diff --git a/README.md b/README.md
index aab39d9..6eea143 100644
--- a/README.md
+++ b/README.md
@@ -124,10 +124,9 @@ The input's file paths and directory structure will be preserved in the [`dag-pb
 - `chunker` (string, defaults to `"fixed"`): the chunking strategy. Supports:
   - `fixed`
   - `rabin`
-- `chunkerOptions` (object, optional): the options for the chunker. Defaults to an object with the following properties:
-  - `avgChunkSize` (positive integer, defaults to `262144`): the average chunk size (rabin chunker only)
-  - `minChunkSize` (positive integer): the minimum chunk size (rabin chunker only)
-  - `maxChunkSize` (positive integer, defaults to `262144`): the maximum chunk size
+- `avgChunkSize` (positive integer, defaults to `262144`): the average chunk size (rabin chunker only)
+- `minChunkSize` (positive integer): the minimum chunk size (rabin chunker only)
+- `maxChunkSize` (positive integer, defaults to `262144`): the maximum chunk size
 - `strategy` (string, defaults to `"balanced"`): the DAG builder strategy name. Supports:
   - `flat`: flat list of chunks
   - `balanced`: builds a balanced tree
@@ -144,6 +143,8 @@ The input's file paths and directory structure will be preserved in the [`dag-pb
 - `cidVersion` (integer, default 0): the CID version to use when storing the data (storage keys are based on the CID, _including_ it's version)
 - `rawLeaves` (boolean, defaults to false): When a file would span multiple DAGNodes, if this is true the leaf nodes will not be wrapped in `UnixFS` protobufs and will instead contain the raw file bytes
 - `leafType` (string, defaults to `'file'`) what type of UnixFS node leaves should be - can be `'file'` or `'raw'` (ignored when `rawLeaves` is `true`)
+- `blockWriteConcurrency` (positive integer, defaults to 10) How many blocks to hash and write to the block store concurrently. For small numbers of large files this should be high (e.g. 50).
+- `fileImportConcurrency` (number, defaults to 50) How many files to import concurrently. For large numbers of small files this should be high (e.g. 50).
 
 [ipld-resolver instance]: https://github.com/ipld/js-ipld-resolver
 [UnixFS]: https://github.com/ipfs/specs/tree/master/unixfs
diff --git a/package.json b/package.json
index e9d2f59..5e5b735 100644
--- a/package.json
+++ b/package.json
@@ -38,33 +38,35 @@
   "homepage": "https://github.com/ipfs/js-ipfs-unixfs-importer#readme",
   "devDependencies": {
     "aegir": "^20.0.0",
-    "async-iterator-buffer-stream": "^1.0.0",
-    "async-iterator-last": "^1.0.0",
     "chai": "^4.2.0",
     "cids": "~0.7.1",
+    "deep-extend": "~0.6.0",
     "detect-node": "^2.0.4",
     "dirty-chai": "^2.0.1",
     "ipfs-unixfs-exporter": "^0.39.0",
     "ipld": "^0.25.0",
     "ipld-in-memory": "^3.0.0",
+    "it-buffer-stream": "^1.0.0",
+    "it-last": "^1.0.0",
     "multihashes": "~0.4.14",
     "nyc": "^14.0.0",
     "sinon": "^7.1.0"
   },
   "dependencies": {
-    "async-iterator-all": "^1.0.0",
-    "async-iterator-batch": "~0.0.1",
-    "async-iterator-first": "^1.0.0",
     "bl": "^4.0.0",
-    "deep-extend": "~0.6.0",
     "err-code": "^2.0.0",
     "hamt-sharding": "~0.0.2",
     "ipfs-unixfs": "^0.2.0",
     "ipld-dag-pb": "^0.18.0",
+    "it-all": "^1.0.1",
+    "it-batch": "^1.0.3",
+    "it-first": "^1.0.1",
+    "it-flat-batch": "^1.0.2",
+    "it-parallel-batch": "1.0.2",
+    "merge-options": "^2.0.0",
     "multicodec": "~0.5.1",
     "multihashing-async": "^0.8.0",
-    "rabin-wasm": "~0.0.8",
-    "superstruct": "^0.8.2"
+    "rabin-wasm": "~0.0.8"
   },
   "contributors": [
     "Alan Shaw <alan.shaw@protocol.ai>",
diff --git a/src/dag-builder/file/balanced.js b/src/dag-builder/file/balanced.js
index 72b3cb8..26fd7a2 100644
--- a/src/dag-builder/file/balanced.js
+++ b/src/dag-builder/file/balanced.js
@@ -1,6 +1,6 @@
 'use strict'
 
-const batch = require('async-iterator-batch')
+const batch = require('it-flat-batch')
 
 async function * balanced (source, reduce, options) {
   yield await reduceToParents(source, reduce, options)
diff --git a/src/dag-builder/file/flat.js b/src/dag-builder/file/flat.js
index c7ba75e..7146d2e 100644
--- a/src/dag-builder/file/flat.js
+++ b/src/dag-builder/file/flat.js
@@ -1,11 +1,11 @@
 'use strict'
 
-const batch = require('async-iterator-batch')
+const batch = require('it-flat-batch')
 
 module.exports = async function * (source, reduce) {
   const roots = []
 
-  for await (const chunk of batch(source, Infinity)) {
+  for await (const chunk of batch(source, Number.MAX_SAFE_INTEGER)) {
     roots.push(await reduce(chunk))
   }
 
diff --git a/src/dag-builder/file/index.js b/src/dag-builder/file/index.js
index f3be403..bfb71a6 100644
--- a/src/dag-builder/file/index.js
+++ b/src/dag-builder/file/index.js
@@ -7,7 +7,8 @@ const {
   DAGNode,
   DAGLink
 } = require('ipld-dag-pb')
-const all = require('async-iterator-all')
+const all = require('it-all')
+const parallelBatch = require('it-parallel-batch')
 
 const dagBuilders = {
   flat: require('./flat'),
@@ -15,46 +16,53 @@ const dagBuilders = {
   trickle: require('./trickle')
 }
 
-async function * buildFile (file, source, ipld, options) {
-  let count = -1
-  let previous
-
+async function * importBuffer (file, source, ipld, options) {
   for await (const buffer of source) {
-    count++
-    options.progress(buffer.length)
-    let node
-    let unixfs
+    yield async () => {
+      options.progress(buffer.length)
+      let node
+      let unixfs
 
-    const opts = {
-      ...options
-    }
+      const opts = {
+        ...options
+      }
 
-    if (options.rawLeaves) {
-      node = buffer
+      if (options.rawLeaves) {
+        node = buffer
 
-      opts.codec = 'raw'
-      opts.cidVersion = 1
-    } else {
-      unixfs = new UnixFS(options.leafType, buffer)
+        opts.codec = 'raw'
+        opts.cidVersion = 1
+      } else {
+        unixfs = new UnixFS(options.leafType, buffer)
 
-      if (file.mtime) {
-        unixfs.mtime = file.mtime
-      }
+        if (file.mtime) {
+          unixfs.mtime = file.mtime
+        }
+
+        if (file.mode) {
+          unixfs.mode = file.mode
+        }
 
-      if (file.mode) {
-        unixfs.mode = file.mode
+        node = new DAGNode(unixfs.marshal())
       }
 
-      node = new DAGNode(unixfs.marshal())
+      const cid = await persist(node, ipld, opts)
+
+      return {
+        cid: cid,
+        unixfs,
+        node
+      }
     }
+  }
+}
 
-    const cid = await persist(node, ipld, opts)
+async function * buildFileBatch (file, source, ipld, options) {
+  let count = -1
+  let previous
 
-    const entry = {
-      cid: cid,
-      unixfs,
-      node
-    }
+  for await (const entry of parallelBatch(importBuffer(file, source, ipld, options), options.blockWriteConcurrency)) {
+    count++
 
     if (count === 0) {
       previous = entry
@@ -149,7 +157,7 @@ const fileBuilder = async (file, source, ipld, options) => {
     throw errCode(new Error(`Unknown importer build strategy name: ${options.strategy}`), 'ERR_BAD_STRATEGY')
   }
 
-  const roots = await all(dagBuilder(buildFile(file, source, ipld, options), reduce(file, ipld, options), options.builderOptions))
+  const roots = await all(dagBuilder(buildFileBatch(file, source, ipld, options), reduce(file, ipld, options), options))
 
   if (roots.length > 1) {
     throw errCode(new Error('expected a maximum of 1 roots and got ' + roots.length), 'ETOOMANYROOTS')
diff --git a/src/dag-builder/file/trickle.js b/src/dag-builder/file/trickle.js
index a970517..9941d87 100644
--- a/src/dag-builder/file/trickle.js
+++ b/src/dag-builder/file/trickle.js
@@ -1,6 +1,6 @@
 'use strict'
 
-const batch = require('async-iterator-batch')
+const batch = require('it-flat-batch')
 
 module.exports = function * trickleReduceToRoot (source, reduce, options) {
   yield trickleStream(source, reduce, options)
diff --git a/src/dag-builder/index.js b/src/dag-builder/index.js
index 013d498..bc2f4b2 100644
--- a/src/dag-builder/index.js
+++ b/src/dag-builder/index.js
@@ -30,13 +30,13 @@ async function * dagBuilder (source, ipld, options) {
         }
       }
 
-      const chunker = createChunker(options.chunker, validateChunks(source), options.chunkerOptions)
+      const chunker = createChunker(options.chunker, validateChunks(source), options)
 
       // item is a file
-      yield fileBuilder(entry, chunker, ipld, options)
+      yield () => fileBuilder(entry, chunker, ipld, options)
     } else {
       // item is a directory
-      yield dirBuilder(entry, ipld, options)
+      yield () => dirBuilder(entry, ipld, options)
     }
   }
 }
diff --git a/src/dir-sharded.js b/src/dir-sharded.js
index e515b8a..97dcf36 100644
--- a/src/dir-sharded.js
+++ b/src/dir-sharded.js
@@ -9,7 +9,7 @@ const multihashing = require('multihashing-async')
 const Dir = require('./dir')
 const persist = require('./utils/persist')
 const Bucket = require('hamt-sharding')
-const extend = require('deep-extend')
+const mergeOptions = require('merge-options').bind({ ignoreUndefined: true })
 
 const hashFn = async function (value) {
   const hash = await multihashing(Buffer.from(value, 'utf8'), 'murmur3-128')
@@ -36,7 +36,7 @@ const defaultOptions = {
 
 class DirSharded extends Dir {
   constructor (props, options) {
-    options = extend({}, defaultOptions, options)
+    options = mergeOptions(defaultOptions, options)
 
     super(props, options)
 
diff --git a/src/index.js b/src/index.js
index 4ff621d..fc09074 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,78 +1,41 @@
 'use strict'
 
-const { superstruct } = require('superstruct')
 const dagBuilder = require('./dag-builder')
 const treeBuilder = require('./tree-builder')
-const mh = require('multihashes')
+const parallelBatch = require('it-parallel-batch')
+const mergeOptions = require('merge-options').bind({ ignoreUndefined: true })
 
-const struct = superstruct({
-  types: {
-    codec: v => ['dag-pb', 'dag-cbor', 'raw'].includes(v),
-    hashAlg: v => Object.keys(mh.names).includes(v),
-    leafType: v => ['file', 'raw'].includes(v)
-  }
-})
-
-const ChunkerOptions = struct({
-  minChunkSize: 'number?',
-  maxChunkSize: 'number?',
-  avgChunkSize: 'number?',
-  window: 'number?',
-  polynomial: 'number?'
-}, {
-  maxChunkSize: 262144,
-  avgChunkSize: 262144,
-  window: 16,
-  polynomial: 17437180132763653 // https://github.com/ipfs/go-ipfs-chunker/blob/d0125832512163708c0804a3cda060e21acddae4/rabin.go#L11
-})
-
-const BuilderOptions = struct({
-  maxChildrenPerNode: 'number?',
-  layerRepeat: 'number?'
-}, {
-  maxChildrenPerNode: 174,
-  layerRepeat: 4
-})
-
-const Options = struct({
-  chunker: struct.enum(['fixed', 'rabin']),
-  rawLeaves: 'boolean?',
-  hashOnly: 'boolean?',
-  strategy: struct.enum(['balanced', 'flat', 'trickle']),
-  reduceSingleLeafToSelf: 'boolean?',
-  codec: 'codec?',
-  format: 'codec?',
-  hashAlg: 'hashAlg?',
-  leafType: 'leafType?',
-  cidVersion: 'number?',
-  progress: 'function?',
-  wrapWithDirectory: 'boolean?',
-  shardSplitThreshold: 'number?',
-  onlyHash: 'boolean?',
-  chunkerOptions: ChunkerOptions,
-  builderOptions: BuilderOptions,
-
-  wrap: 'boolean?',
-  pin: 'boolean?',
-  recursive: 'boolean?',
-  ignore: 'array?',
-  hidden: 'boolean?',
-  preload: 'boolean?'
-}, {
+const defaultOptions = {
   chunker: 'fixed',
-  strategy: 'balanced',
+  strategy: 'balanced', // 'flat', 'trickle'
   rawLeaves: false,
+  onlyHash: false,
   reduceSingleLeafToSelf: true,
   codec: 'dag-pb',
   hashAlg: 'sha2-256',
-  leafType: 'file',
+  leafType: 'file', // 'raw'
   cidVersion: 0,
   progress: () => () => {},
-  shardSplitThreshold: 1000
-})
+  shardSplitThreshold: 1000,
+  fileImportConcurrency: 50,
+  blockWriteConcurrency: 10,
+  minChunkSize: 262144,
+  maxChunkSize: 262144,
+  avgChunkSize: 262144,
+  window: 16,
+  polynomial: 17437180132763653, // https://github.com/ipfs/go-ipfs-chunker/blob/d0125832512163708c0804a3cda060e21acddae4/rabin.go#L11
+  maxChildrenPerNode: 174,
+  layerRepeat: 4,
+  wrapWithDirectory: false,
+  pin: true,
+  recursive: false,
+  ignore: null, // []
+  hidden: false,
+  preload: true
+}
 
 module.exports = async function * (source, ipld, options = {}) {
-  const opts = Options(options)
+  const opts = mergeOptions(defaultOptions, options)
 
   if (options.cidVersion > 0 && options.rawLeaves === undefined) {
     // if the cid version is 1 or above, use raw leaves as this is
@@ -93,10 +56,10 @@ module.exports = async function * (source, ipld, options = {}) {
   }
 
   if (options.format) {
-    options.codec = options.format
+    opts.codec = options.format
   }
 
-  for await (const entry of treeBuilder(dagBuilder(source, ipld, opts), ipld, opts)) {
+  for await (const entry of treeBuilder(parallelBatch(dagBuilder(source, ipld, opts), opts.fileImportConcurrency), ipld, opts)) {
     yield {
       cid: entry.cid,
       path: entry.path,
diff --git a/src/tree-builder.js b/src/tree-builder.js
index 55bab49..cd43170 100644
--- a/src/tree-builder.js
+++ b/src/tree-builder.js
@@ -5,7 +5,7 @@ const flatToShard = require('./flat-to-shard')
 const Dir = require('./dir')
 const toPathComponents = require('./utils/to-path-components')
 const errCode = require('err-code')
-const first = require('async-iterator-first')
+const first = require('it-first')
 
 async function addToTree (elem, tree, options) {
   const pathElems = toPathComponents(elem.path || '')
@@ -61,6 +61,9 @@ async function * treeBuilder (source, ipld, options) {
   }, options)
 
   for await (const entry of source) {
+    if (!entry) {
+      continue
+    }
     tree = await addToTree(entry, tree, options)
 
     yield entry
diff --git a/test/benchmark.spec.js b/test/benchmark.spec.js
index b0b1db2..fae3f48 100644
--- a/test/benchmark.spec.js
+++ b/test/benchmark.spec.js
@@ -5,8 +5,8 @@ const importer = require('../src')
 
 const IPLD = require('ipld')
 const inMemory = require('ipld-in-memory')
-const bufferStream = require('async-iterator-buffer-stream')
-const all = require('async-iterator-all')
+const bufferStream = require('it-buffer-stream')
+const all = require('it-all')
 
 const REPEATS = 10
 const FILE_SIZE = Math.pow(2, 20) * 500 // 500MB
diff --git a/test/builder-balanced.spec.js b/test/builder-balanced.spec.js
index f064dd8..17242a3 100644
--- a/test/builder-balanced.spec.js
+++ b/test/builder-balanced.spec.js
@@ -5,7 +5,7 @@ const chai = require('chai')
 chai.use(require('dirty-chai'))
 const expect = chai.expect
 const builder = require('../src/dag-builder/file/balanced')
-const all = require('async-iterator-all')
+const all = require('it-all')
 
 function reduce (leaves) {
   if (leaves.length > 1) {
diff --git a/test/builder-dir-sharding.spec.js b/test/builder-dir-sharding.spec.js
index 057eb2e..b52b07b 100644
--- a/test/builder-dir-sharding.spec.js
+++ b/test/builder-dir-sharding.spec.js
@@ -9,8 +9,8 @@ chai.use(require('dirty-chai'))
 const expect = chai.expect
 const IPLD = require('ipld')
 const inMemory = require('ipld-in-memory')
-const all = require('async-iterator-all')
-const last = require('async-iterator-last')
+const all = require('it-all')
+const last = require('it-last')
 
 describe('builder: directory sharding', () => {
   let ipld
diff --git a/test/builder-flat.spec.js b/test/builder-flat.spec.js
index 1e3acc8..e3f0339 100644
--- a/test/builder-flat.spec.js
+++ b/test/builder-flat.spec.js
@@ -5,7 +5,7 @@ const chai = require('chai')
 chai.use(require('dirty-chai'))
 const expect = chai.expect
 const builder = require('../src/dag-builder/file/flat')
-const all = require('async-iterator-all')
+const all = require('it-all')
 
 function reduce (leaves) {
   if (leaves.length > 1) {
diff --git a/test/builder-only-hash.spec.js b/test/builder-only-hash.spec.js
index 45ac5dd..e7e7642 100644
--- a/test/builder-only-hash.spec.js
+++ b/test/builder-only-hash.spec.js
@@ -7,7 +7,7 @@ const expect = chai.expect
 const IPLD = require('ipld')
 const inMemory = require('ipld-in-memory')
 const builder = require('../src/dag-builder')
-const all = require('async-iterator-all')
+const all = require('it-all')
 
 describe('builder: onlyHash', () => {
   let ipld
@@ -30,18 +30,14 @@ describe('builder: onlyHash', () => {
       format: 'dag-pb',
       hashAlg: 'sha2-256',
       wrap: true,
-      chunkerOptions: {
-        maxChunkSize: 1024
-      },
-      builderOptions: {
-        maxChildrenPerNode: 254
-      }
+      maxChunkSize: 1024,
+      maxChildrenPerNode: 254
     }))
 
     expect(nodes.length).to.equal(1)
 
     try {
-      await ipld.get(nodes[0].cid)
+      await ipld.get((await nodes[0]()).cid)
 
       throw new Error('Should have errored')
     } catch (err) {
diff --git a/test/builder-trickle-dag.spec.js b/test/builder-trickle-dag.spec.js
index 8e625b4..ba6c239 100644
--- a/test/builder-trickle-dag.spec.js
+++ b/test/builder-trickle-dag.spec.js
@@ -5,7 +5,7 @@ const chai = require('chai')
 chai.use(require('dirty-chai'))
 const expect = chai.expect
 const builder = require('../src/dag-builder/file/trickle')
-const all = require('async-iterator-all')
+const all = require('it-all')
 
 const createValues = (max) => {
   const output = []
diff --git a/test/builder.spec.js b/test/builder.spec.js
index 1eea492..bbeb9d0 100644
--- a/test/builder.spec.js
+++ b/test/builder.spec.js
@@ -9,7 +9,7 @@ const IPLD = require('ipld')
 const inMemory = require('ipld-in-memory')
 const UnixFS = require('ipfs-unixfs')
 const builder = require('../src/dag-builder')
-const first = require('async-iterator-first')
+const first = require('it-first')
 
 describe('builder', () => {
   let ipld
@@ -27,9 +27,7 @@ describe('builder', () => {
     format: 'dag-pb',
     hashAlg: 'sha2-256',
     progress: () => {},
-    chunkerOptions: {
-      maxChunkSize: 262144
-    }
+    maxChunkSize: 262144
   }
 
   it('allows multihash hash algorithm to be specified', async () => {
@@ -45,7 +43,7 @@ describe('builder', () => {
         content: Buffer.from(content)
       }
 
-      const imported = await first(builder([inputFile], ipld, options))
+      const imported = await (await first(builder([inputFile], ipld, options)))()
 
       expect(imported).to.exist()
 
@@ -76,7 +74,7 @@ describe('builder', () => {
         content: Buffer.alloc(262144 + 5).fill(1)
       }
 
-      const imported = await first(builder([Object.assign({}, inputFile)], ipld, options))
+      const imported = await (await first(builder([inputFile], ipld, options)))()
 
       expect(imported).to.exist()
       expect(mh.decode(imported.cid.multihash).name).to.equal(hashAlg)
@@ -96,7 +94,7 @@ describe('builder', () => {
         content: null
       }
 
-      const imported = await first(builder([Object.assign({}, inputFile)], ipld, options))
+      const imported = await (await first(builder([Object.assign({}, inputFile)], ipld, options)))()
 
       expect(mh.decode(imported.cid.multihash).name).to.equal(hashAlg)
 
diff --git a/test/chunker-fixed-size.spec.js b/test/chunker-fixed-size.spec.js
index e61653e..276702a 100644
--- a/test/chunker-fixed-size.spec.js
+++ b/test/chunker-fixed-size.spec.js
@@ -6,7 +6,7 @@ const chai = require('chai')
 chai.use(require('dirty-chai'))
 const expect = chai.expect
 const isNode = require('detect-node')
-const all = require('async-iterator-all')
+const all = require('it-all')
 const loadFixture = require('aegir/fixtures')
 const rawFile = loadFixture((isNode ? __dirname : 'test') + '/fixtures/1MiB.txt')
 
diff --git a/test/chunker-rabin.spec.js b/test/chunker-rabin.spec.js
index 6caccc8..9f9a4af 100644
--- a/test/chunker-rabin.spec.js
+++ b/test/chunker-rabin.spec.js
@@ -7,7 +7,7 @@ chai.use(require('dirty-chai'))
 const expect = chai.expect
 const loadFixture = require('aegir/fixtures')
 const isNode = require('detect-node')
-const all = require('async-iterator-all')
+const all = require('it-all')
 
 const rawFile = loadFixture((isNode ? __dirname : 'test') + '/fixtures/1MiB.txt')
 
diff --git a/test/hash-parity-with-go-ipfs.spec.js b/test/hash-parity-with-go-ipfs.spec.js
index ce936be..94e44fd 100644
--- a/test/hash-parity-with-go-ipfs.spec.js
+++ b/test/hash-parity-with-go-ipfs.spec.js
@@ -9,7 +9,7 @@ const expect = chai.expect
 const IPLD = require('ipld')
 const inMemory = require('ipld-in-memory')
 const randomByteStream = require('./helpers/finite-pseudorandom-byte-stream')
-const first = require('async-iterator-first')
+const first = require('it-first')
 
 const strategies = [
   'flat',
diff --git a/test/import-export-nested-dir.spec.js b/test/import-export-nested-dir.spec.js
index 8d58080..ae60712 100644
--- a/test/import-export-nested-dir.spec.js
+++ b/test/import-export-nested-dir.spec.js
@@ -6,7 +6,7 @@ chai.use(require('dirty-chai'))
 const expect = chai.expect
 const IPLD = require('ipld')
 const inMemory = require('ipld-in-memory')
-const all = require('async-iterator-all')
+const all = require('it-all')
 const importer = require('../src')
 const exporter = require('ipfs-unixfs-exporter')
 
diff --git a/test/importer.spec.js b/test/importer.spec.js
index 6fab88e..321db3a 100644
--- a/test/importer.spec.js
+++ b/test/importer.spec.js
@@ -17,8 +17,8 @@ const loadFixture = require('aegir/fixtures')
 const isNode = require('detect-node')
 const bigFile = loadFixture((isNode ? __dirname : 'test') + '/fixtures/1.2MiB.txt')
 const smallFile = loadFixture((isNode ? __dirname : 'test') + '/fixtures/200Bytes.txt')
-const all = require('async-iterator-all')
-const first = require('async-iterator-first')
+const all = require('it-all')
+const first = require('it-first')
 
 function stringifyMh (files) {
   return files.map((file) => {
@@ -563,9 +563,7 @@ strategies.forEach((strategy) => {
 
       const options = {
         progress: spy(),
-        chunkerOptions: {
-          maxChunkSize
-        }
+        maxChunkSize
       }
 
       await all(importer([{