PERF: groupby.transform skip fast path when only a single group exists

doc/source/whatsnew/v1.5.0.rst

@@ -205,6 +205,7 @@ Performance improvements
 - Performance improvement in :meth:`.GroupBy.transform` for some user-defined DataFrame -> Series functions (:issue:`45387`)
 - Performance improvement in :meth:`DataFrame.duplicated` when subset consists of only one column (:issue:`45236`)
 - Performance improvement in :meth:`.GroupBy.transform` when broadcasting values for user-defined functions (:issue:`45708`)
+- Performance improvement in :meth:`.GroupBy.transform` for user-defined functions when only a single group exists (:issue:`44977`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/groupby/generic.py

@@ -1208,6 +1208,11 @@ def _choose_path(self, fast_path: Callable, slow_path: Callable, group: DataFram
         path = slow_path
         res = slow_path(group)
 
+        if self.ngroups == 1:
+            # no need to evaluate multiple paths when only
+            # a single group exists
+            return path, res
+
         # if we make it here, test if we can use the fast path
         try:
             res_fast = fast_path(group)

pandas/tests/groupby/transform/test_transform.py

@@ -480,7 +480,7 @@ def test_transform_coercion():
     # 14457
     # when we are transforming be sure to not coerce
     # via assignment
-    df = DataFrame({"A": ["a", "a"], "B": [0, 1]})
+    df = DataFrame({"A": ["a", "a", "b", "b"], "B": [0, 1, 3, 4]})
     g = df.groupby("A")
 
     expected = g.transform(np.mean)