Hi Snail and Ren!
Sorry for the performance issues with the game. We are making gradual steps with each build to improve performance. In 0.7.0, enemies that are not on camera are no longer rendered, their script calls have also been reduced.
Stages are still intensive but we are planning to take chunks of it out of the renderer too. It should run fine on most gaming PC builds but we want it to work on as many systems as possible without sacrificing sprite quality.