The GHz doesn't matter in this case. If the operating system has a higher-priority interrupt then your incoming interrupt is going to get delayed by a random amount. The whole purpose is accurate timing, which is not possible when the computer has other jobs to do.
And that's before you consider multitasking, which will add all sorts of delays to unload and load register contexts.
If you're on the wrong on-ramp on the freeway interchange then a Ferrari will get you to the correct one very fast, but it would be quicker if you had got on the correct on-ramp in the first place.