Deriving instances is slow
Let's take this file (from haskell-src-exts) and build it with -O0. On my machine it takes 55s.
If we remove deriving of all classes except Functor (which is the only one used by the module itself), the time will drop down to 4s.
Different classes vary in their compile-time cost, but there's no single culprit. Among the costly ones are Generic, Data, Show, Ord.
haskell-src-exts users are very annoyed by its long compile time, about 10 minutes, especially because we don't compile with -O0 by default. It would be nice to get this fixed.