I'm optimizing performance of PBT generation and shrinking in [elm-test](https://github.com/elm-explorations/test/compare/master...ja...) - on its own PBT-heavy test suite I got it down from 1336ms to 891ms by using JS TypedArrays.

I'm also experimenting with coverage-guided PBT input generation in the same library, AFL-style -- right now elm-test only has random input generation.