It is unlikely the branch prediction heuristics that is the problem (that in the Intel's microcode). The problem is in the mitigations necessary to make it imp

It is unlikely the branch prediction heuristics that is the problem (that in the Intel's microcode). The problem is in the mitigations necessary to make it impossible/more difficult to exploit these side channel attacks. And that is costly because memory and needs to be moved around constantly. So that adds a ton of extra overhead whenever a context switch is made.