adding whatsnew note

Author: realead

doc/source/whatsnew/v1.2.0.rst

@@ -284,6 +284,7 @@ Performance improvements
 - ``Styler`` uuid method altered to compress data transmission over web whilst maintaining reasonably low table collision probability (:issue:`36345`)
 - Performance improvement in :meth:`pd.to_datetime` with non-ns time unit for ``float`` ``dtype`` columns (:issue:`20445`)
 - Performance improvement in setting values on a :class:`IntervalArray` (:issue:`36310`)
+- Performance improvement in :meth:`pd.DataFrame.groupby` for ``float`` ``dtype`` (:issue:`28303`)
 
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