Why performance mods matter
Vanilla Minecraft's rendering is not well optimized. Performance mods can realistically double or triple your FPS. That means smoother gameplay, higher render distance without lag, and the ability to run more mods without your computer choking.
The Fabric stack
Fabric has the best performance mods, and they all work together. Install all of these:
Sodium
The single most important mod. It rewrites Minecraft's entire rendering engine. Expect 2-5x FPS improvement depending on your system. Also adds better graphics settings menus.
Lithium
Optimizes game physics, mob AI, and world generation. Works silently in the background without changing any gameplay. Reduces server-side lag too.
FerriteCore
Reduces memory usage significantly. If you play modpacks, this is essential. Less RAM used means fewer garbage collection pauses (those annoying stutters).
Extra optimizations
ModernFix patches various memory leaks and loading issues. Entity Culling stops rendering mobs you can't see. ImmediatelyFast speeds up GUI and text rendering. Krypton optimizes networking for multiplayer. More Culling extends culling to particles and block entities.
You don't need all of these, but they help. Add them if you want every bit of performance.
Forge options
Most Fabric performance mods have Forge ports. Embeddium is Sodium for Forge. Radium is Lithium for Forge. FerriteCore works on both. OptiFine is the classic all-in-one, but it conflicts with many mods and usually performs worse than Sodium-based alternatives now.
Don't want to pick mods yourself?
Install a performance modpack instead. Fabulously Optimized bundles everything with OptiFine-like features. Simply Optimized is minimal and focused. These give you the full performance stack without manual setup.
Settings that help
Beyond mods, tweak your video settings. Render distance of 8-12 chunks is enough for most gameplay. Use Fast graphics instead of Fancy. Turn particles to Minimal. Disable clouds or set them to Fast. Turn off VSync unless you're getting screen tearing.
System-wise: update your graphics drivers, close other apps while playing, use fullscreen mode, and disable overlays from Discord, Steam, etc.
Compatibility notes
Sodium and OptiFine don't work together. Choose one. Most other performance mods play nice with each other.