Minecraft 1.17 Unplayable Lag when upgrading existing 1.16.5 worlds to 1.17

When I upgrade an existing world(saved in 1.16.5 for me) to 1.17, there is unbearable lag, so much so that I can't play the game at all. However, when I create a new world in 1.17, there is absolutely no lag whatsoever, and the game runs buttery smooth. And the world that was upgraded from 1.16.5 to 1.17 was also running buttery smooth in 1.16.5, but in 1.17, it's absolutely unplayable.