A massive amount of tick lag when moving vertically in the server. Whether falling or flying in creative mode, vertical movement causes massive lag spikes that can reach an avg tick of 600ms or more. This causes the server to repeatedly say "Can't keep up!" — and "Player moved too quickly!" when falling — before eventually crashing.

Crash log, server log and screenshot are from the same test. This occurs every time. I initiate it by flying up and dropping down in a 3x3 tunnel from the surface to diamond level repeatedly. (This is an amplified world)