Gameplay stutters and you walk like in treacle

Multiplayer. LAN server at work which I also access from home along with another friend who uses Macs.

When I arrive at a certain co-ordinate my FPS drops from 60-70 down to a reported 5-10 however functionally it is much less. The screen locks for a second every other second. Turns become 90 degree jumps. All animations ~(water, torches) lock for a second. As I struggle away my character walks in thick treacle (even when the lock-up is not locked up) which slowly speeds up as I get further away. I never noticed this issue in any previous versions although I may not have been to this location for a long while.

Problem occurs along the same X axis at any Y axis and over a long-ish Z axis e.g. I have seen it happen at 310,54,27 and also 311,65,-22. Not encountered it anywhere else yet.

Issue is easy to re-create simply by teleporting to 310,54,27 on my World.

(Access available if necessary to trusted developers)

Smooth lighting is off. View distance has no effect.