A full replacement of the Minecraft lighting engine with an extendable API, shipping with Phosphor as a reference implementation.
The main performance improvement comes from the fact that the lighting engine is evaluated "lazily", thus it is able to batch more lighting updates into a single pass.
Here are some simple to understand values, generated via profiling a pre-set test case:
Profiler is started right before clicking Create New World, and stopped after 10 seconds of flying in a straight line in creative mode.
Without Lumi, the active server time spent in lighting engine is 34% of the total active server time (not counting when the server thread is sleeping).
With Lumi, this fraction goes down to 13%, or an approximately 2.5x performance increase over vanilla.
Horizontal axis is time, vertical axis is the call stack. The blue highlighted parts are calls to the lighting engine update method.
!Flame Graph Without Lumi
!Flame Graph With Lumi
enablePhosphor to false in the
ArchaicFix config.fix_heightmap_range feature is already implemented in Lumi's lighting engine. Set it to false in the
CoreTweaks config.Both of these configs will be automatically modified by Lumi automatically when you launch the game, so you shouldn't even need to edit the config files manually.

Improves world generation speed, reduces server lag, and fixes lag spikes on client by optimizing the Minecraft lighting engine. Based on Phosphor.