The reason the sweeper is running on the main thread is to execute finalizers. If there were no finalizers that had to be run on the main thread this could easily moved on a background thread.

(Summary from various chats)

One way to evaluate this first is to get a ratio between objects that require finalizers and those that don't.

Depending on that, it might be a good idea to enqueue finalizers somewhere and run them on the main thread when the sweeper is finished. This way it should also be possible to measure time spent in finalizers because timing overhead will likely not dominate executing *all* finalizers.

For completeness, Desktop number that we track:
  https://chromeperf.appspot.com/report?sid=cc4dc8aff106735fda60770abfa8798e054eca00d15ecca420e0f16810c13afc

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