PERF: 5x speedup for read_json() with orient='index' by avoiding transpose

Author: qwhelan

pandas/io/json/_json.py

@@ -1085,9 +1085,11 @@ def _parse_no_numpy(self):
             self.check_keys_split(decoded)
             self.obj = DataFrame(dtype=None, **decoded)
         elif orient == "index":
-            self.obj = DataFrame(
-                loads(json, precise_float=self.precise_float), dtype=None
-            ).T
+            self.obj = DataFrame.from_dict(
+                loads(json, precise_float=self.precise_float),
+                dtype=None,
+                orient="index",
+            ).sort_index(axis="columns")
         elif orient == "table":
             self.obj = parse_table_schema(json, precise_float=self.precise_float)
         else: