I did a quick test on the base project (the same one from the demo available for download). I changed the size of the available area and the distance, and I didn't notice any significant FPS drops. Could you compare this behavior with what you're experiencing? I'll carefully check the distance issue.
Thank you very much for your responses and testing this.
I'm guessing your computer is quite a bit better than mine. The default number of regional tiles in the example project slowed mine down to 10-20 FPS, but reducing it to 8-12 regional tiles keeps my FPS high, so it's probably just a matter of me having an older computer. I may dive into the code eventually to see if there's a way I can simplify the fish pathfinding or something.