I came across a very annoying bug when trying to find a jungle biome (implying they are too rare, but this is off topic):

While beyond about 115.000-120.000 blocks from the centre of the world, falling block entities (falling sand, gravel, concrete powder, anvil, etc.) begin to increasingly terribly move in the air, teleport, lag, jerk all over the screen and worse (when at about the world limit).

This effect gets worse and worse the further you are from x=0,z=0 and it does occur in pre-17w13b worlds (I also tested it in a 1.8 world) and it does occur in worlds created in 17w13b and other recent updates.

It is generally also brought with low speeds and lag, even if you are in a completely new and empty world with just a few chunks loaded.

To recreate:
1. Open any world in 17w13b from any version, or, create a new world.
2. Teleport at least 120.000 (120km) blocks away from (x=0,z=0) (preferably 1000km or more for the best results)
3. Let a gravity affected block fall from any distance to get the effect (take about 4m or more to better see it in action)

It works in both multiplayer and singleplayer worlds, but the effect is a lot less active on superflat worlds (go at least 1000km from the centre to get visable effects)

I hope you are able to resolve this issue!
Good luck and thanks for the great work