This mod moves Minecraft's light engine processing to another thread, "fixing" [MC-162253](https://bugs.mojang.com/browse/MC-162253) (underlying performance issue still exists, but does not impact FPS anymore).
[MC-123584](https://bugs.mojang.com/browse/MC-123584) is also fixed by simply moving all rendering off the main thread. It is recommended to enable the Forge "alwaysSetupTerrainOffThread" option to avoid rendering issues.