Raspberry pi 4B is very weak specially on the GPU where the heavy load is, the only thing you can do is disable shaders, I would start with the CRT effect, if that's not enough or you are not happy with the results try reducing resolution even more. If you want to see other love2d games that use shaders for ARM you can download plenty of them from my repository for sailfish os. I optimized many games and most of the gain always come from reducing resolution (you do not need to reduce the resolution of the whole games, you can internally render elements at different resolution, background elements should have less priority).