Updating to 1.21 causes severe lag

After downloading the latest update (1.21) a few hours ago, my game has experienced severe lag from delayed input (taking a few seconds after pressing the W-key to walk forward, or crossbows not being able to reload) to projectiles like arrows and snowballs being in really slow motion after firing.

This in stark contrast to when I was on the last version of 1.20 half a day ago, when everything was functioning perfectly lag-free. 

At first I thought it was because I had too many ticking areas active (I had 5/10 active, with each of them affecting 1-3 chunks in my superflat world), so I removed all of them, but it didn't work. 

Then I tried loading into another regular world to see if the problem persists, which it still does, but not as bad as my superflat world.