Use a higher value for connecion_pool_maxsize.

configuration.py

@@ -20,6 +20,7 @@
 
 from __future__ import absolute_import
 
+import multiprocessing
 import logging
 import sys
 
@@ -83,11 +84,12 @@ def __init__(self):
         # Set this to True/False to enable/disable SSL hostname verification.
         self.assert_hostname = None
         # urllib3 connection pool's maximum number of connections saved
-        # per pool. Increasing this is useful for cases when you are
-        # making a lot of possibly parallel requests to the same host,
-        # which is often the case here.
-        # When set to `None`, will default to whatever urllib3 uses
-        self.connection_pool_maxsize = None
+        # per pool. urllib3 uses 1 connection as default value, but this is
+        # not the best value when you are making a lot of possibly parallel
+        # requests to the same host, which is often the case here.
+        # cpu_count * 5 is used as default value to increase performance
+        # This is used because it's the default value for ThreadPoolExecutor
+        self.connection_pool_maxsize = multiprocessing.cpu_count() * 5
         # http proxy setting
         self.http_proxy_url = None