(OP here)

I do say it's not that big of a deal in the end: it almost always is OK and at the end you optimize the inner loops by looking at profiles; like in any other project.

But when using GHC, I have indeed sometimes ran into situations where I expect something to be fast when it is not (e.g., `ByteString.map (+ value)` is incredibly slow compared to a pseudo-C loop).

I also did find a bona fides performance bug in GHC https://ghc.haskell.org/trac/ghc/ticket/11783