Investigate the performance impact of hs-boot files
GHC cannot inline through hs-boot imports. If a hot code path uses an hs-boot import, this fact may drastically impact performance.
In particular, the pattern synonyms One and Many use eqType via an hs-boot import. This just might be a problem that would manifest as an increase in time spent but not an increase in allocations, which is a problem we saw when optimizing the linear types implementation.
There may be other such cases, and I think time trawling through {-# SOURCE #-} imports would be well spent.