PERF: creating string Series/Arrays from sequence with many strings (#36304)

Author: topper-123

doc/source/whatsnew/v1.2.0.rst

@@ -205,6 +205,7 @@ Deprecations
 Performance improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
+- Performance improvements when creating Series with dtype `str` or :class:`StringDtype` from array with many string elements (:issue:`36304`)
 - Performance improvement in :meth:`GroupBy.agg` with the ``numba`` engine (:issue:`35759`)
 -
 

pandas/_libs/lib.pyx

@@ -655,6 +655,10 @@ cpdef ndarray[object] ensure_string_array(
 
     for i in range(n):
         val = result[i]
+
+        if isinstance(val, str):
+            continue
+
         if not checknull(val):
             result[i] = str(val)
         else: