AFAIK, intel microcode patching mechanism works in parallel with normal instruction execution. That is, the instructions are decoded and executed normally, and

AFAIK, intel microcode patching mechanism works in parallel with normal instruction execution. That is, the instructions are decoded and executed normally, and in parallel different circuitry looks for known fault cases, and traps if necessary. So long as you won't find the special case, the execution takes exactly as long. And there is not enough time in the universe to iterate through a 128-bit number, so you won't find the special case.