I’m not sure that performance needs to constrain web engine design the way it traditionally has. The world has changed since the browser perf war started.

Just try disabling the JIT in a modern browser and you’ll find your UX is not really any worse.

HW has gotten faster than when the current browser perf wars kicked off. So it might be that to have a meaningfully useful browser engine alternative, perf won’t be the top concern. Compatibility will though, just because of the sheer number of features a modern browser engine supports.

And if you don’t have JIT, then it’s not clear if Spectre is even a meaningful concern.