In most games, textures typically don't affect performance, until you run out of vram, which will cause either major stutters or the game to crash. That being said, in Minecraft if I use a 512x resourcepack with shaders, the resourcepack does cause an additional 15-20% performance loss, despite not running out of vram. 256x there's no difference. This probably depends on PC specs. If you want to improve performance check out Sodium, there's a lot of other performance mods too, such as Entity Culling, FerriteCore, ModernFix, Lithium, C2ME, Noisium, etc.
