diff --git a/jedi/evaluate/compiled/subprocess/__init__.py b/jedi/evaluate/compiled/subprocess/__init__.py
index 9cc8704a..6de4521e 100644
--- a/jedi/evaluate/compiled/subprocess/__init__.py
+++ b/jedi/evaluate/compiled/subprocess/__init__.py
@@ -142,6 +142,9 @@ class _CompiledSubprocess(object):
             args,
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
+            # Use system default buffering on Python 2 to improve performance
+            # (this is already the case on Python 3).
+            bufsize=-1
         )
 
     def run(self, evaluator, function, args=(), kwargs={}):