Use defaultdict for optimization (pandas-dev#32278)

Author: jaketae

pandas/core/frame.py

@@ -8446,9 +8446,8 @@ def isin(self, values) -> "DataFrame":
 
 def _from_nested_dict(data):
     # TODO: this should be seriously cythonized
-    new_data = {}
+    new_data = collections.defaultdict(dict)
     for index, s in data.items():
         for col, v in s.items():
-            new_data[col] = new_data.get(col, {})
             new_data[col][index] = v
     return new_data