I experience this but only when my computer is using 100% disk (hooray for windows 10) or when I don’t reboot my computer for a full couple of days.
This may or may not be your problem, but try rebooting and maybe checking out how much your other programs are eating resources.
EDIT: This could also possibly be a result of that particular game world calculating a lot of things at once. If, say, you had experimental z-levels (known to cause lag, if I remember right) and were in, say, a big apartment building swarming with zombies.
Does this happen for all your worlds or just one?