The default implementations of foldl1 and foldr1 in Data.Foldable were spine-strict, probably by accident. Make them lazy.
Make default implementations somewhat less right-handed.
Use explicit lambdas in fold helpers to (hopefully) help with inlining and (certainly) increase clarity.
Remove redundant (and illegal) constraints on the class variable that crept in as copypasta.
Use (safe) coercions to improve efficiency of some newtype wrapping/unwrapping, as suggested by Edward Kmett.