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 Modrinth - 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. Some mods may require Indium to be compatible with Sodium. - By CaffeineMC

Lithium Modrinth - 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

FerriteCore CurseForge Modrinth - Memory usage optimizations - By malte0811

ImmediatelyFast CurseForge Modrinth - Improves the immediate mode rendering performance of the client. - By RaphiMC

ModernFix CurseForge Modrinth - All-in-one mod that improves performance, reduces memory usage, and fixes many bugs. - By embeddedt

MoreCulling Modrinth - A mod that changes how multiple types of culling are handled in order to improve performance. - By FX

Enhanced Block Entities CurseForge Modrinth - Reduce FPS lag with block entities, as well as customize them with resource packs. Keeps chest opening animation. - By FoundationGames

FastChest GitHub - Removes dynamic models from chests and making them render as static chunk geometry like normal blocks. This means they will lose their lid opening animation. Redundant with Enhanced Block Entities. - By FakeDomi

Starlight CurseForge Modrinth - Fabric mod for rewriting the light engine to fix lighting performance and lighting errors. Incompatible with Phosphor. Redundant client side as of Minecraft 1.20. - By PaperMC, Spottedstar

Phosphor CurseForge Modrinth - Phosphor is a Minecraft mod which works to optimize one of game's most inefficient areas– the lighting engine. Incompatible with Starlight. Redundant as of Minecraft 1.20. - By CaffeineMC

Cull Less Leaves CurseForge Modrinth - Instead of leaving just the outer layer of leaves, Cull Less Leaves also renders a certain amount of layers defined in the config. - By isxander

MoreCulling Modrinth - A mod that changes how multiple types of culling are handled in order to improve performance. - By FX

Cull Leaves CurseForge Modrinth - Adds culling to leaf blocks, providing a huge performance boost over vanilla. - By Motschen

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

Canvas Renderer CurseForge Modrinth - 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

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

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

Zume CurseForge Modrinth - A simple zoom mod - By Nolij

FabriZoom Modrinth - Highly-configurable zoom for Fabric/NeoForge. Maintained fork of OkZoomer v4 - By trainb0y

Just Zoom CurseForge Modrinth - Zoom by pressing a hotkey and adjust the zoom factor with your mouse wheel! - By keksuccino

Entity Model Features CurseForge Modrinth - EMF is an OptiFine format Custom Entity Model replacement mod available for Fabric and Forge. - By traben_0

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

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

Continuity CurseForge Modrinth - Provides OptiFine parity for resource packs that use the OptiFine CTM format. It also supports OptiFine-format emissive textures for block and item models. 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

MoreMcmeta CurseForge Modrinth - Animate almost any Minecraft texture with JSON and OptiFine files. Texture configuration API. Emissive textures are available with plugin. - By soir20

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

Polytone CurseForge Modrinth - Customize Map Color, Block Colors, Colormaps and Block Sounds, Biome Colors, Dye Colors. Supports OptiFine format. For Resource Packs. - By MehVahdJukaar

Vanadium Modrinth - A simple naive implementation for custom colors in resource packs with caching and blending. Supports OptiFine format. - By AluTheCrow

Colormatic CurseForge Modrinth - 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 CurseForge Modrinth - Implements Custom Skyboxes like OptiFine using a custom format. - By AMereBagatelle, FlashyReese

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

LambDynamicLights CurseForge Modrinth - 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

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

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

Boosted Brightness CurseForge Modrinth - Set your brightness beyond default levels - By adamviola

Full Brightness Toggle Modrinth CurseForge - Press G to toggle the brightness/gamma in-game to maximum and get night vision. - By Serilum

Colormatic CurseForge Modrinth - 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

BetterGrassify CurseForge Modrinth - OptiFine's Fancy and Fast better grass implemented on Fabric and NeoForge! - By UltimatChamp

Sodium Extra CurseForge Modrinth - Features that shouldn't be in Sodium. Sodium Extra aims to bring most of OptiFine's eye-candy options to Sodium - By FlashyReese

Puzzle Modrinth CurseForge - Adds resourcepack features and a GUI to more conveniently configure OptiFine alternatives.

OptiGUI CurseForge Modrinth - Custom inventory GUIs with OptiFine resource pack support & many more features - By opekope2