This issue is related to the Frames per second on the server itself.
John and Eddie are constantly trying to improve the server so lets hope this issue is fixed over time.
Choppy movement was caused by the server FPS but this is not the case at the moment (FPS has been steady for a week)
According to John the movement speed should show a bit of variation. They should still be able to reach you before dying though.
Just heard someone else about the speed so i will go check it out (after my round of replies here )
It wasn't choppy movement, i've seen choppy movement when the mob suddenly appears a lot closer that it did before, this is slow moving mobs, where you can mostly just walk backwards and they do not reach you.
Ive made a youtube video of the slow moving mobs.
This time it was the Spiders (the normal ones, not the green ones).
They just don't seem to run at you when they realise they have been shot at. They move towards you slightly faster than walking and to the right a bit, not straight. All the spiders when this happens you can out run them by running backwards and you do not get attacked.
Sorry about the crappy video quality, but you should be able to see what I mean about them not reaching you before they die. The video was made today (28th September 2013, at around 4am UK time.)