postMessage uses it transparently: https://developer.mozilla.org/en-US/docs/Web/API/Worker/post...
Benchmarks aren’t showing structuredClone favorably versus JSON round tripping. Likely because JSON-compatible data structures are less complex than clonable data. I suspect with this change JSON will now be faster than structuredClone.
Which ones?
Structured clone seems to be approximately the same as FF according to this (structuredClone is slightly faster on my machine, probably margin of error): https://measurethat.net/Benchmarks/Show/23052/0/structuredcl... (Linux: FF 141, Chromium 138).