Why does FPS drop from 100 to 5 with custom islands (mod)?

Back to playing Stranded Deep. The game was running fine, without crashes, with fps that reached over 100.
But it was just going to a custom island (mod) that I added, the fps plummeted, oscillating between a measly 5 to 9 fps. Is there any solution to resolve this? Or no chance for the game to work with custom mods?