Don't most machines with this kind of CPU in them not run Windows?

If you’re talking about the i9-9900, it’s a very high clock frequency part with “only” 8 cores and part of the consumer line (no ecc support). I’d actually think most people who have it run Windows and use it for things like gaming.

Most maybe, but there’s still quite a large fleet of windows servers out there.

i've seen js performance on synthetic benchmarks drop somewhat over the past months. perhaps JS JITs like V8 rely to some degree on specific branch prediction properties.