diff --git a/Cargo.lock b/Cargo.lock
index 3175cd557286..e12de77020a2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3297,7 +3297,7 @@ dependencies = [
 [[package]]
 name = "webrender"
 version = "0.39.0"
-source = "git+https://github.com/servo/webrender#4ab9b9b00b00c93a51469775f56c53eeea68b5ab"
+source = "git+https://github.com/servo/webrender#c003d553bdd856e1e78d213375582a1f2b091a06"
 dependencies = [
  "app_units 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "bincode 1.0.0-alpha6 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3326,7 +3326,7 @@ dependencies = [
 [[package]]
 name = "webrender_traits"
 version = "0.39.0"
-source = "git+https://github.com/servo/webrender#4ab9b9b00b00c93a51469775f56c53eeea68b5ab"
+source = "git+https://github.com/servo/webrender#c003d553bdd856e1e78d213375582a1f2b091a06"
 dependencies = [
  "app_units 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "bincode 1.0.0-alpha6 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/python/servo/testing_commands.py b/python/servo/testing_commands.py
index 427f0fad38f8..23b25a00e2aa 100644
--- a/python/servo/testing_commands.py
+++ b/python/servo/testing_commands.py
@@ -458,6 +458,14 @@ def run_test_list_or_dispatch(self, requested_paths, correct_suite, correct_func
     def wptrunner(self, run_file, **kwargs):
         self.set_software_rendering_env(kwargs['release'])
 
+        # By default, Rayon selects the number of worker threads
+        # based on the available CPU count. This doesn't work very
+        # well when running tests on CI, since we run so many
+        # Servo processes in parallel. The result is a lot of
+        # extra timeouts. Instead, force Rayon to assume we are
+        # running on a 2 CPU environment.
+        os.environ['RAYON_RS_NUM_CPUS'] = "2"
+
         os.environ["RUST_BACKTRACE"] = "1"
         kwargs["debug"] = not kwargs["release"]
         if kwargs.pop("chaos"):