What is Folia?
Folia is a new fork of Paper that adds regionized multithreading to the server. Access to Folia builds isn't currently available. To build Folia, you need to compile from source following Paper's standard compilation guide.
How's it different from Paper?
Folia is designed to address the constant bottleneck of the Minecraft server running on a single thread causing performance issues. It is by no means a drop-in replacement for Paper.
Is Folia for me?
Folia won't be useful for the majority of servers out there and will break most public plugins. It's ideal for servers with very high players counts, that offer gamemodes that naturally spread players out, such as SkyBlock or SMP.
Where can I learn more?
You can visit the GitHub repository for Folia to read an overview of the project, and some frequently asked questions.