PERF: Optimising Series.nunique() for NaN values pandas-dev#40865

Author: Kenil Mehta

pandas/core/base.py

@@ -1040,8 +1040,11 @@ def nunique(self, dropna: bool = True) -> int:
         >>> s.nunique()
         4
         """
-        obj = remove_na_arraylike(self) if dropna else self
-        return len(obj.unique())
+        uniqs = self.unique()
+        if dropna:
+            return (~np.isnan(uniqs)).sum()
+        else:
+            return len(uniqs)
 
     @property
     def is_unique(self) -> bool: