My understanding is that there are several forms of microcode updates. Sometimes the microcode can do simple patches like inserting a nop between problematic i

My understanding is that there are several forms of microcode updates. Sometimes the microcode can do simple patches like inserting a nop between problematic instruction combinations. This usually doesn't have much (if any) negative performance implications. The instruction decoder (again AFAIK) is fairly programmable. However if there is a more serious flaw in the actual silicon the microcode must rewrite the problem instructions to emulate the correct behavior and that can make it extremely slow - think emulating floating point but not quite as bad.