Having looked at ghcide, it always calls these methods via hashWithSalt. Having looked at hashable, it does appear that if you don't implement hashWithSalt then it's a generic method, which means in both cases it walks the FilePath, and avoids all your clever precomputing the Int trick. Much better to properly implement hashWithSalt, and it does seem to have some value of directly implementing hash.

I haven't profiled the code to confirm it is a speed up, but it seems highly likely!