Basically grows the vanilla threadcount/list sizes for everything that could make chunk compilation/rendering faster, as well as removing the limitations in those functions that could make the chunk loading stop in favor of other rendering elements
The mod name is technically a bit clickbaity, Minecraft's chunkloading is already multithreaded, except:
Its intended (and from what I can think of only) purpose is to basically let you crank up your render distance to 32+ and load everything in a solo map at once (eg to take a big screenshot), instead of having to wait 25 years for everything to load with the vanilla settings.
Mineplex Backup Project, altho a bit late I need to take screenshots of EVERY map, and not having to wait for the maps to load for too much time is a real time saver
Optifine makes the overall loading slower. Still partially works tho.

READ THE DESCRIPTION BEFORE DOWNLOADING. A legacy Forge mod that supercharges the chunk loading, even if it tanks the FPS.