In bug 1717631 I added telemetry for structured cloning usage to see if it popped out as an optimization target, but it didn't really show much. But I've also heard of examples where it was observed to be involved in slow behavior, so I'm creating a meta bug to be able to record such cases. (I'm not currently planning on optimizing anything here, but I might if the dependent bugs say it's important.)

Note that this is filed within the JS component since that's where the core of the algorithm is implemented, but various non-JS types are implemented in the DOM code.