When the player gets above a certain altitude (around 180-190ish blocks) all the entities and block updates etc. get really slow and stuttery. The log also starts getting spammed with "Can't keep up! Is the server overloaded? Running ****ms or ** ticks behind" constantly. Really frustrating for building high up in the world. It also logged the error attached to the report (only once though). Moving the player around is otherwise very smooth and placing blocks is fine. It's just entities, water flow, and tile entities.