You can't actually change time, just your perception of time.

If you can't perceive anything while time is frozen as a consequence of biological structure, then your "side-effect" is just complete nullification. You wouldn't be changing your perception of time at all. Not much of a side-effect.

If your brain and senses can act at super speed but everything else in your body acts in real time, you'd still have an advantage in reaction time as you would be able to recognize and initiate your first reaction near-instantly.