Krypton is a Fabric mod that attempts to optimize the Minecraft networking stack. It derives from work done in the Velocity and Tuinity projects.
These are unofficial ports of Andrew's original mod to versions that aren't maintained or present upstream. 1.16.1 is the current priority.
Krypton derives itself from Ancient Greek kryptos, which means "the hidden one". This makes it evident most of the benefit from Krypton is "hidden" but is noticeable by a server administrator.
The wiki contains important information – read it. For support you can contact me at MrMangoHands#8467 on discord, or open an issue. Join Andrew's Discord to discuss the mod.
CAUTION! No guarantees are provided about Krypton's stability, compatibility with other mods, ability to be used on every server, or support for every possible setup out there. Support for this mod is provided on a "best-effort" basis. You have been warned.
Releases can be found here.
You can also compile the mod from source in the usual fashion.