I agree with MarkT - what you need is a discrete part with the necessary speed to do the converting of the pulse to a pin transition - since presumably you need to do more than that, you could then use an Arduino alongside it, to do the other stuff (and enable/disable that part, etcetera).
I don't think this calls for a Pi, since you don't need fast processing, just fast reaction to a pulse, which is easy to do with a separate IC or two.