As of 21w06a, there is tremendous FPS lag. I am averaging below 20 fps when previously rarely see below 100 fps. There are not really lag spikes, it is more just general lag. And it isn't chunk loading lag, as they seem to load almost normally for me, it is just rendering FPS lag.

This occurred when I was spectating the caves underground in spectator mode, though I would assume it would occur with any game mode.

I'm fairly certain this is caused by the new world generation in one way or another.