try

requirements.txt

@@ -11,3 +11,5 @@
 -r requirements/requirements-documentation.txt
 -r requirements/requirements-codestyle.txt
 -r requirements/requirements-packaging.txt
+
+ujson==2.0.3

rest_framework/renderers.py

@@ -102,7 +102,7 @@ class JSONRenderer(BaseRenderer):
             separators = INDENT_SEPARATORS
 
         ret = json.dumps(
-            data, cls=self.encoder_class,
+            data,
             indent=indent, ensure_ascii=self.ensure_ascii,
             allow_nan=not self.strict, separators=separators
         )

rest_framework/utils/json.py

@@ -9,7 +9,7 @@ handled by users at the renderer and parser layer.
 from __future__ import absolute_import
 
 import functools
-import json  # noqa
+import ujson as json  # noqa
 
 
 def strict_constant(o):