add NEWS

Author: ben-schwen

NEWS.md

@@ -350,6 +350,8 @@ See [#2611](https://github.com/Rdatatable/data.table/issues/2611) for details. T
 
 25. By-group operations on missing rows (e.g. `foo[c(i, NA), bar, by=grp]`) now avoid leaving in data from the previous groups, [#7442](https://github.com/Rdatatable/data.table/issues/7442). Thanks @aitap for the report and the fix.
 
+26. Grouping by a factor with many groups is now fast again, fixing a timing regression introduced in [#6890](https://github.com/Rdatatable/data.table/pull/6890) where UTF-8 coercion and level remapping were performed unnecessarily, [#7404](https://github.com/Rdatatable/data.table/issues/7404). Thanks @ben-schwen for the report and fix.
+
 ### NOTES
 
 1. The following in-progress deprecations have proceeded: