Document - Default ElasticSearch performance expectation

[owaincuvelier]

It would be ideal to set out a rough estimate of what a default ElasticSearch Cluster can handle in terms of performance. This can be measured in a few ways, maybe as the total duration of index completion, or as requests/pm.

We should also note in the documentation the impact of additional metadata and database writes as both can have a huge impact on ES cluster health/performance.