When changing a resource pack; the game becomes super laggy and slow. The FPS of  Minecraft version 1.20.2 drops A LOT and this cannot be fixed by restarting the launcher, restarting the game, or restarting your device. You must fully reinstall Minecraft 1.20.2 and sometimes for modded clients reinstall the whole launcher.

P.S. This CANNOT be fixed by switching back to the vanilla resource pack.