diff --git a/buildbot/buildbot_json_magic_substitutions.py b/buildbot/buildbot_json_magic_substitutions.py
index 37f93be..b608f9f 100644
--- a/buildbot/buildbot_json_magic_substitutions.py
+++ b/buildbot/buildbot_json_magic_substitutions.py
@@ -357,6 +357,15 @@
       if gpu.startswith('10de'):
         return ['--jobs=1']
 
+  # trace_test flakily hangs Win NVIDIA GTX 1660 machines crbug.com/406454932.
+  # Speculatively disable parallelism to check if it is related.
+  is_trace_test = (test_name.startswith('trace_test')
+                   or test_config.get('telemetry_test_name') == 'trace_test')
+  if os_type == 'win' and is_trace_test:
+    for gpu in _GetGpusFromTestConfig(test_config):
+      if gpu.startswith('10de:2184'):
+        return ['--jobs=1']
+
   if os_type in ['lacros', 'linux', 'mac', 'win']:
     return ['--jobs=4']
   return ['--jobs=1']
diff --git a/buildbot/client.v8.fyi.json b/buildbot/client.v8.fyi.json
index 185fae1..9429790 100644
--- a/buildbot/client.v8.fyi.json
+++ b/buildbot/client.v8.fyi.json
@@ -3028,7 +3028,7 @@
           "--stable-jobs",
           "--extra-browser-args=--enable-logging=stderr --js-flags=--expose-gc",
           "--enforce-browser-version",
-          "--jobs=4"
+          "--jobs=1"
         ],
         "merge": {
           "script": "//testing/merge_scripts/standard_isolated_script_merge.py"