I started a hardcore world in 21w44a, fresh. This was done in Windows 10. After playing for a little while, I saved and exited. The next day, I opened up the same world. Framerate tanked, getting worse the more of the world was loaded. Eventually, Minecraft completely hung and I had to kill the process. 

I copied the world to a different machine (running macOS) and was able to reproduce the problem. Including having to kill the process. It was using over 500% CPU at the time it was killed. Logs don't show anything interesting, but are attached. 

I've uploaded a zip file of the world in question to dropbox, as other worlds haven't been able to reproduce this, it might be something specific in the world data itself, or where the player exited and re-enters the world.

https://www.dropbox.com/s/f71bf7qspiy2pjv/New%20World%20%285%29.zip?dl=0