perf2?

Author: h-vetinari

pandas/core/strings.py

@@ -56,10 +56,10 @@ def str_cat_core(array, sep):
     if sep == '':
         return array.sum(axis=1)
     else:
-        tmp = np.full((array.shape[0], 2 * array.shape[1] - 1),
-                      fill_value=sep, dtype='object')
-        tmp[:, ::2] = array
-        return tmp.sum(axis=1)
+        res = array[:, 0]
+        for i in range(1, array.shape[1]):
+            res += sep + array[:, i]
+        return res
 
 
 def _na_map(f, arr, na_result=np.nan, dtype=object):