update whatsnew

Author: auderson

doc/source/whatsnew/v3.0.0.rst

@@ -596,6 +596,7 @@ Performance improvements
 - Performance improvement in ``DataFrameGroupBy.__len__`` and ``SeriesGroupBy.__len__`` (:issue:`57595`)
 - Performance improvement in indexing operations for string dtypes (:issue:`56997`)
 - Performance improvement in unary methods on a :class:`RangeIndex` returning a :class:`RangeIndex` instead of a :class:`Index` when possible. (:issue:`57825`)
+- Performance improvement in :meth:`DataFrame.astype` when converting to extension floating dtypes, e.g. "Float64" (:issue:`60066`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.bug_fixes: