Does a comparable metaphor exist for cognition that exists for slow & fast twitch muscle fibers?
It sounds like you are talking about system 1 and 2 thinking that Kahneman called'fast' and 'slow'.

While it's analagous to muscle fibre I'm not convinced it is as physiologically based. 

System 1 is fast, but also characterised by its unconscious nature. The decisions made are automatic, and based on mental shortcuts or heuristics. It serves us well a lot of the time but is more error prone. 

System 2 is more conscious and requires more cognitive effort. We are more likely to use it for complex decisions but only if we are aware of the complexity and consequences. It generally produces more reliable decisions.

You'll find the same concept with different labels, but the fast and slow thinking that Kahneman referred to is probably the best known.