!XyleneOptimal
Minecraft uses sin() and cos() math functions constantly (lighting, rendering, entity movement, etc.). Normally these are calculated fresh every time. Xylene pre-calculates all 65,536 possible answers and stores them in a table. When the game needs sin/cos, it just looks up the answer instantly instead of doing the math.
Minecraft stores a LOT of data in things called HashMaps (think of them like dictionaries). Xylene swaps out the default Java ones for specially optimized ones that are faster for the types of data Minecraft uses (NBT data, entity tracking, etc.)
Every tick (20 times per second), Minecraft updates every single entity — zombies, cows, item frames, everything. Xylene skips updating entities that are more than 48 blocks away from any player. They're too far away for you to notice anyway. (Bosses and players are never skipped.)
Minecraft constantly checks if entities are colliding with things. Xylene tracks whether an entity has actually moved. If it hasn't moved, it skips the collision check entirely — no point rechecking something that's standing still.
When your game talks to the server, numbers are encoded in a special format called "VarInt." Xylene replaces the encoding loop with a faster hand-written version.
It shows all the toggle switches for every Xylene optimization category, so you can turn individual optimizations on/off.
smooth entity/player motion between ticks at any framerate.
chunk geometry compiled off the main thread.
GL shaders pre-compiled on startup to kill first-use stutter.
pack switching no longer freezes the game.
AO values cached, recomputed only when blocks change.
simplified pathfinding for densely packed entities.
blends out small server position corrections.
caps packet drain time per frame to protect the render thread.
interpolates small server snaps; large ones still apply instantly.
short buffer that absorbs brief ping spikes.
!Xylene
About 40+ other mixins are registered but are currently empty placeholders, things like hopper optimization, redstone optimization, lighting improvements, chunk loading tweaks, etc. The framework is there but the actual optimization code hasn't been filled in yet. They load harmlessly and do nothing.

A Small Optimization Mod that will help boost your FPS. Won't be massive but its noticeable.