Today at work I narrowed down a nasty interaction between ormolu and GHC 9.4 (since we recently upgraded everything):
Apparently by default fourmolu uses TemplateHaskell to compile a large map at compile time as an optimization. This worked on previous versions of GHC and then on GHC 9.4 it suddently leads to a massive slowdown.
One of the things I love about my new job is that we're given the time and the encouragement to really dig into problems and push fixes or report issues upstream.