Hey Jare, how are you?
Recently I've noticed that some video cards aren't rendering the water effect correctly. This is causing polygon buildup and a general drop in FPS. I can try to diagnose if this is exactly your case, but to try and solve it more comprehensively, I'll release a version of the plugin tomorrow where you can disable the water effect and use RPG Maker's own textures and effects. This should improve performance and maintain a high frame rate. I'll post a video later showing the behavior in the test project, and we can discuss which solution best suits you.
What do you think?
Thank you for your message and all your support. I guarantee we'll solve your problem and find the best solution.