It seems a few problems arose soon after DoctorWhen spawned in an ender dragon. Server has been laggy for most everyone it seems, plus spawn seems a little less protected from mobs...
And the reason I suspect it (the cause of the lag) being the ender dragon spawned is that, moments after DoctorWhen died in the end, but before he went back in, the enderman walked at normal speed (it had been going incredibly slow prior to When's death).
(Sorry the images are so large, I don't know how to make them smaller.)