adding whatsnew

Author: realead

doc/source/whatsnew/v1.3.0.rst

@@ -250,6 +250,7 @@ Performance improvements
 - Performance improvement in :meth:`Series.isin` for nullable data types (:issue:`38340`)
 - Performance improvement in :meth:`DataFrame.corr` for method=kendall (:issue:`28329`)
 - Performance improvement in :meth:`core.window.Rolling.corr` and :meth:`core.window.Rolling.cov` (:issue:`39388`)
+- Performance improvement in :func:`unique` for object data type (:issue:`37615`)
 
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