PERF: Improve performance when accessing GroupBy.groups (#53088)

* PERF: Improve performance when accessing GroupBy.groups

* Update v2.1.0.rst

* Fix

Author: phofl

doc/source/whatsnew/v2.1.0.rst

@@ -299,6 +299,7 @@ Performance improvements
 - Performance improvement in :meth:`DataFrame.loc` when selecting rows and columns (:issue:`53014`)
 - Performance improvement in :meth:`Series.add` for pyarrow string and binary dtypes (:issue:`53150`)
 - Performance improvement in :meth:`DataFrame.transpose` when transposing a DataFrame with a single masked dtype, e.g. :class:`Int64` (:issue:`52836`)
+- Performance improvement in :meth:`.DataFrameGroupBy.groups` (:issue:`53088`)
 - Performance improvement in :meth:`DataFrame.loc` when selecting rows and columns (:issue:`53014`)
 - Performance improvement in :meth:`Series.corr` and :meth:`Series.cov` for extension dtypes (:issue:`52502`)
 - Performance improvement in :meth:`Series.str.get` for pyarrow-backed strings (:issue:`53152`)