OptiFine tends to cause incompatibilities with other mods and may be less effective than some of the alternatives listed below. OptiFine also doesn't officially support running on Fabric (Though you can use the OptiFabric mod for OptiFine compatibility.)

Sodium - Sodium is a free and open-source rendering engine replacement for the Minecraft client which greatly improves frame rates and stuttering while fixing many graphical issues. - By CaffeineMC

Lithium - Lithium is a general-purpose optimization mod for Minecraft which works to improve a number of systems (game physics, mob AI, block ticking, etc) without changing any behavior. - By CaffeineMC

Starlight - Fabric mod for rewriting the light engine to fix lighting performance and lighting errors. - By PaperMC, Spottedstar

Phosphor - Phosphor is a Minecraft mod which works to optimize one of game's most inefficient areas– the lighting engine. - By CaffeineMC

Iris - An Open Source shader mod compatible with OptiFine shader packs. Compatible with Sodium. - By IrisShaders

Canvas Renderer - Canvas is a shader-based Renderer for the Fabric modding toolchain. It supports all features of the proposed Fabric Rendering API plus extensions defined in FREX. Incompatible with Sodium. - By vram-guild, GrondagTheBarbarian

WI Zoom - The zoom from the Wurst Client as a standalone mod. - By Wurst Imperium

Logical Zoom - This is a simple Minecraft Fabric mod that adds a configurable zoom key. - By LogicalGeekBoy

Zoomify - A zoom mod with infinite customizability. - By XanderIsDev

Custom Entity Models (CEM) - An implementation of custom entity models heavily based off of OptiFine's format that aims to achieve feature parity with OptiFine's custom entity models. - By dorianpb

Entity Texture Features - Adds many new Entity Texture Features, including entity and player skin features. - By traben_0

CIT Resewn - MCPatcher's CIT rewritten outside of OptiFine as a standalone mod for Fabric. - By SHsuperCM

Chime - Adds a predicate system for more dynamic and interesting item model variation. It supports the ability to vary model based on countless conditions, such as the item's name, enchantments, NBT, dimension, entity, and plenty more. Chime is built in with vanilla's predicate system, building on it instead of replacing it with a foreign system. Incompatible with MCPatcher/OptiFine's CIT. - By EmilyPloszaj

Continuity - It is designed to provide full OptiFine parity for all resource packs that use the OptiFine CTM format. Continuity also supports OptiFine-format emissive textures for block and item models. Continuity depends on the Fabric API and is client-side only. It includes two built-in resource packs - one provides default connected textures, similar to OptiFine, and the other provides a fix for glass pane culling. - By Pepper_Bell

Animatica - A mod implementing the OptiFine/MCPatcher animated texture format. - By FoundationGames

Colormatic - Colormatic is an independent implementation that is resource-pack-compatible with the original Custom Colors mod for Minecraft on Fabric. It is fully compatible with resource packs that use Optifine's custom colors feature. Colormatic sports almost all features of the original Custom Colors mod and adds a few of its own. - By kwertiTheCats

FabricSkyboxes - Implements Custom Skyboxes like Optifine using a custom format. - By AMereBagatelle, FlashyReese

FabricSkyboxes Interop - FabricSkyBoxes Interoperability for MCPatcher/OptiFine Skies. - By FlashyReese

Mini Dynamic Lights - A fast dynamic light mod by doing minimal computation. - By jfudson/ye-yu

LambDynamicLights - This mod adds dynamic lights to Minecraft. Dynamic lights are lights created by an entity holding an item which makes light as a block, or created by an entity on fire, etc. - By LambdAurora

Colormatic - Colormatic is an independent implementation that is resource-pack-compatible with the original Custom Colors mod for Minecraft on Fabric. It is fully compatible with resource packs that use Optifine's custom colors feature. Colormatic sports almost all features of the original Custom Colors mod and adds a few of its own. Allows use of fullbright resource packs. - By kwertiTheCats

Gamma Utils - Gamma / brightness / night vision mod, making it easy to see in the dark. Basically fullbright for Fabric. - By Sjouwer