What performance mods actually do
Vanilla Minecraft's chunk rendering is single-threaded and dated. Performance mods replace or patch the slow parts: better chunk meshing, fewer redundant memory allocations, multithreaded tasks where it's safe. On most systems you'll see a clear FPS gain; exact numbers depend on your CPU, GPU, render distance, and how many mods you've stacked on top.
The core trio (Fabric / Quilt / NeoForge)
Sodium
Replaces the chunk renderer. Far and away the biggest FPS win in most setups. Adds a redesigned video-settings screen with the new options.
Lithium
Optimizes game logic, entity AI, hopper interactions, block ticks. No gameplay changes. Helps both clients and servers.
FerriteCore
Cuts memory use by deduplicating block-state and model data. Smaller heap means shorter and less frequent GC pauses, which is what most "lag spikes" actually are.
Useful extras
ModernFix bundles a long list of memory and startup patches. Entity Culling skips rendering entities whose bounding boxes aren't visible. ImmediatelyFast speeds up text and GUI rendering. Krypton trims networking overhead in multiplayer. More Culling extends culling to block entities and particles.
You don't need all of these. Add the ones that target your specific bottleneck.
Forge and NeoForge equivalents
Most of the Fabric stack has been ported. Embeddium is the Forge/NeoForge fork of Sodium. Radium is Lithium for Forge/NeoForge. FerriteCore and ModernFix already support all four loaders. OptiFine still exists, but it conflicts with many modern mods and Sodium-based stacks generally outperform it now.
If you'd rather not pick mods individually
Install a performance-only modpack. Fabulously Optimized bundles Sodium + Iris + a long tail of patches and aims to match or beat OptiFine in features. Simply Optimized is a smaller alternative. Both are on Modrinth.
In-game settings that help
Render distance 8–12 chunks is plenty for normal play. Fast graphics, particles Minimal, clouds Fast or Off. Disable VSync unless you see tearing. Sodium adds its own performance tab with extra toggles.
System-side: update GPU drivers, close other heavy apps, and disable overlays (Discord, Steam, GeForce Experience), they regularly cost a few FPS.
Compatibility notes
Sodium and OptiFine can't run together, they hook the same parts of the renderer. Iris requires Sodium. Most other performance mods compose fine with each other.