diff --git a/zuul.d/base.yaml b/zuul.d/base.yaml
index 8644b41..cc1479b 100644
--- a/zuul.d/base.yaml
+++ b/zuul.d/base.yaml
@@ -116,3 +116,4 @@
         ADVANCED_INSTANCE_TYPE: ds512M
         ADVANCED_INSTANCE_USER: ubuntu
         BUILD_TIMEOUT: 784
+      tempest_concurrency: 3  # out of 4