Sadly that wouldn't help much since lots of important stuff happens on the same thread(s) that run JS. You also have the issue of javascript from website 1 tryi

Sadly that wouldn't help much since lots of important stuff happens on the same thread(s) that run JS. You also have the issue of javascript from website 1 trying to steal data from website 2, in that case pinning them both to your JS core wouldn't actually protect you from the attack. Among other things there are now mechanisms for doing page rasterization (PaintWorklet) and sound synthesis (AudioWorklet) in JS, so the overhead involved in making all your JS share a single core becomes more dramatic. There's also lots of stuff out there that uses Shared Workers and Workers to do background computation and that won't be background anymore if you pin them all to your JS core.