Afterburner: Optimized

What it improves:

Batched chunk drawing: each 128x64x128 block region is drawn with one call instead of one call per chunk section.
Smaller chunk vertices: 16 bytes per vertex instead of 28, so the graphics card reads less for every block face. Chunks also take less graphics memory.
Occlusion culling: chunks hidden behind nearer terrain (hills, caves, walls) aren't drawn. Works on OpenGL and Vulkan.
Entity culling: mobs, chests, signs and other entities hidden behind terrain aren't drawn.
Face culling: block faces that point away from the camera are skipped.
Translucent face culling: the same for water, glass and ice.
Faster draw prep: skips the per-frame work for chunk layers that have nothing to draw.
Off-screen chunk building: chunks around you get built while the builders have time, not only once you look at them, so turning around shows fewer holes.
Cheaper texture filtering: smooth filtering reads each texture 4 times per pixel instead of 8 or 9. The picture stays the same.

Smaller chunk buffers: GPU memory for chunks grows as needed instead of being reserved 128 MB at a time. On laptops with built-in graphics, that memory is your normal RAM.
Freed build copies: chunks let go of the copy of the blocks they were built from. Vanilla keeps one per chunk.
Memory trim: while the game is paused or in the background, Java gives unused memory back to the system.

Parallel world generation: trees, ores and structures are placed on every CPU core instead of one.
Faster lighting: light updates remember the sections they're working in instead of looking them up for every block. It uses the same algorithm as vanilla, so the result is the same.
Faster collisions: less work per block when checking what moving entities bump into.
Faster mob AI: mob brains and searches for beds, job blocks and similar use plain arrays and loops instead of hash maps and streams.
Leaner chunk sections: less memory for every block and biome container.

Lower worker priority: background workers run just below normal priority, so frames come first when every core is busy.