What's New entry.

docs/src/whatsnew/latest.rst

@@ -30,7 +30,12 @@ This document explains the changes made to Iris for this release
 ✨ Features
 ===========
 
-#. N/A
+#. `@trexfeathers`_ added a new :class:`~iris.Future` flag -
+   ``date_microseconds`` - which sets whether Iris should use the new
+   microsecond-precision units (see :class:`cf_units.Unit`) when the unit
+   is a time unit. The previous maximum precision was seconds. You should check
+   your code for new floating point problems if activating this (e.g. when
+   using the :class:`~iris.Constraint` API). (:pull:`6260`)
 
 
 🐛 Bugs Fixed
@@ -50,7 +55,10 @@ This document explains the changes made to Iris for this release
 🚀 Performance Enhancements
 ===========================
 
-#. N/A
+#. Note that due to the new ``date_microseconds`` :class:`~iris.Future` flag,
+   the time coordinate categorisation speedup introduced in
+   :doc:`/whatsnew/3.11` will only be available when
+   ``iris.FUTURE.date_microseconds == True``.
 
 
 🔥 Deprecations