Disabling HT can't be done per-process. Since per-frame browser processes need to be protected from one another, you can't even run two browser processes on the same core in HT. And scheduling a browser process while leaving the second HT slot on the same core empty is even slower than turning HT off.

Browsers rely on kernel/hardware mitigations. Google people even claimed it's impossible to fully mitigate those vulnerabilities in existing Javascript implementation in Chrome.

It's the job of the OS to make sure one process can't read the memory of another process. What you're saying is that should now be the responsibility of the JavaScript engine as well as the OS.