Fabric No Lag is a bare bones client sided only modpack designed to get the most FPS out of Minecraft with the least lag possible, using highly optimised mods.
All video settings are set to the absolute lowest settings by default to provide the best FPS out of the box.
Once you are in the game turn up your video settings to your liking (preferably starting with view distance first).
Mods in this modpack optimise things like better entity/block culling for more fps, faster lighting calculations, cached game assets for faster loading, memory usage optimisations/fixes, optimised networking packets, and other core game mechanic optimisations (game physics, mob AI, block ticking, etc). This is all achieved without majorly changing the core game, so everything should still work and play like the vanilla game, just with better overall performance.