late reply but this module I found basically solved this issue. I suggest anyone else looking into Roblox fragment shaders to use it, it’s pretty well written and uses parallel luau to speed things up a good amount. Link to RbxShader v1.8.5
I had to edit it quite a bit to make it compatible for my game but its an incredibly useful module otherwise.
edit: I should also mention that the module has solutions for shader interlacing, which is why its so performant