--- a/taskcluster/ci/test/tests.yml
+++ b/taskcluster/ci/test/tests.yml
@@ -1548,16 +1548,21 @@ web-platform-tests:
             - --test-type=testharness
 
 web-platform-tests-reftests:
     description: "Web platform reftest run"
     suite: web-platform-tests-reftests
     treeherder-symbol: tc-W(Wr)
     max-run-time: 5400
     instance-size: xlarge
+    chunks:
+      by-test-platform:
+        macosx.*: 1
+        windows.*: 1
+        default: 6
     e10s:
         by-test-platform:
             macosx.*: true
             default: both
     docker-image: {"in-tree": "desktop1604-test"}
     run-on-projects:
         by-test-platform:
             windows10.*: ['mozilla-central', 'try']