[sliverstorm]

I believe you can make a process noncacheable today, and maybe even disable branch prediction. This would totally shut down Spectre and Meltdown. You can disable SMT, and there's a whole host of other things you can do to isolate your "secure" process on an existing chip. Nobody has done these things because they like performance.

For most of what most people do on their computers most of the time, performance is fine without speculative execution or branch prediction

I think you underestimate the importance of branch prediction.