As a huge Haskell proponent: this is a totally legitimate question, sorry you're being downvoted.

Writing extremely performant Haskell is a very specialized skill. Happily, Haskell is still extremely fast even without fine optimizations.

It depends on what you want to do: if you're writing a moon lander and don't know anything about GHC internals you may be overreaching yourself=) But for most things like web apps etc. knowing the basics is enough.