Basically, as far as I can tell, the client expects the tick rate (set with /tick or otherwise) to be 20TPS or less when updating physics. This results in buggy behavior when physics are updated client-side, then retrieved from the internal/external server (I tested this on both a singleplayer world and a multiplayer world and the behavior appears to be the same):

• Player moves as if the tick rate was still 20 TPS, whereas player physics are scaled down when TPS is <20

• • Edit: This movement behavior applies to clouds as well.

• Certain entities teleport a certain amount depending on TPS (possibly because entity position packets are sent every 0.5 seconds despite the game expecting 20TPS max? Idk)

• • Confirmable entities include any kind of item and any kind of falling block (including the Dragon Egg & Anvil)

• • Note that this issue does NOT arise with any of the following (as far as my testing goes): Mobs (including the Ender Dragon), water/lava, TNT, or arrows

• The daylight cycle is stuttery as it updates the time on some interval, but the client is still trying to update the cycle at 20TPS

How to reproduce

The player movement is kind of a weird technical thing so I am not sure how I could word the steps for reproduction. As for the other two:

Item Teleportation

1. Set the tick rate above 20 with /tick rate <any number above 20>.

1. Drop an item from a reasonable distance (or even from the ground)

1. The item should move as if the game was running at 20TPS for a brief moment before teleporting to its correct position.

Falling Block Teleportation

1. Set the tick rate above 20 with /tick rate <any number above 20>.

1. Place any block that can fall in the air (or summon a falling block of any kind)

1. The block should fall as if the game was running at 20TPS for a brief moment before teleporting to its correct position.

Stuttering Daylight Cycle

1. Set the tick rate above 20 with /tick rate <any number above 20>.

1. Look at the sun/moon.