Not a duplicate of MC-219639 because my issue started in 1.17-pre1 not 21w19a or 21w10a.

In 1.17-pre1 I get about 20 fps when in 21w20a I was getting 60+ fps. The measurements are in the same world.

There is also considerably more tick lag than in 21w20a. Sometimes blocks reappear after I break them and I get stuck on nonexistent blocks whilst swimming (not ghost blocks, this is a different issue). This was all in a singleplayer survival world.

The only cause that I can think of for this is the new advancement triggers, but I may be completely off base with that guess.

I have noticed that this lag comes and goes quite a bit. It also seems to have something to do with how long the world has been open for, as once the chunks have loaded it seems to behave normally for about and hour before the performance severely starts to decline (not very quickly but if you don't restart the game it will drop to 5 fps and you won't be able to interact with the world normally because of server-side lag)