Modded Minecraft can strain even the most powerful computers. Optifine is a mod that changes how Minecraft works and renders things at a more fundamental level than most mods. Because of this, it can provide serious performance improvements, but it can also make things stop working the way they should. I think everyone should at least try Optifine for a bit and see if it helps them. I wrote a guide to help you download and install it, and even uninstall it if you don’t like it.