I think you are confusing meltdown (a speculation attack on hardware permission checks which was patched in later revisions of intel silicon and never affected

I think you are confusing meltdown (a speculation attack on hardware permission checks which was patched in later revisions of intel silicon and never affected other vendors) with Spectre, a general family of attacks on speculative execution, which are generally unsolved. You could of course add dedicated hardware to lower the overhead specifically of memory access permission checks. In fact most CPUs already do, it is called an MMU.