CIT Resewn is required for e.g. the Hypixel+ resource pack to work without Optifine. However it can be VERY laggy when looking at certain items.
The author has a patch pending, but currently seems to be on a hiatus, so until they return this will at least allow playing HyPixel Skyblock without tanking your FPS.
Will be deleted once CIT Resewn is updated.