added explanation

pandas/core/series.py

@@ -2078,6 +2078,9 @@ def searchsorted(self, value, side='left', sorter=None):
         if sorter is not None:
             sorter = ensure_platform_int(sorter)
         if not is_extension_type(self._values):
+            # numpy searchsorted is only fast if value is of same dtype as the
+            # searched array. Below we ensure that value has the right dtype,
+            # and is not 0-dimensional.
             value = np.asarray(value, dtype=self._values.dtype)
             value = value[..., np.newaxis] if value.ndim == 0 else value