I'm not going to take this since I'm not planning to do anything here atm :) The patch makes use do black marking on things which otherwise would use gray marking. So ok, that might mean a tiny overhead, but there aren't that many custom element definitions. The issue was more that the definitions are keeping lots of js alive. And yes, I am surprised to see any perf changes here.
Bug 1628327 Comment 5 Edit History
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I'm not going to take this since I'm not planning to do anything here atm :) (I am looking into other similar CC optimizations atm) The patch makes use do black marking on things which otherwise would use gray marking. So ok, that might mean a tiny overhead, but there aren't that many custom element definitions. The issue was more that the definitions are keeping lots of js alive. And yes, I am surprised to see any perf changes here.