diff --git a/tox.ini b/tox.ini
index 5915acf..d9a410c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,7 +7,7 @@ setenv =
 
 commands =
   python -m git_review.tests.prepare
-  python setup.py testr --slowest --testr-args='{posargs}'
+  python setup.py testr --slowest --testr-args='--concurrency=2 {posargs}'
 
 deps =
     -r{toxinidir}/requirements.txt