Yeah, I had to get up from the computer for something urgent and I didn't come back for about 20 minutes.
How far away, I'm standing right outside the perimeter where they 'load' into my entity count, and every time I check it (for about the past half-hour) it still spikes just as high as it did before. I've logged on to different servers/moved away but it's still PC-cripplingly high.
Unfortunately despawning of mobs does not occur. You will have to find a way to kill them, or beg ICC to reset the mobs on that server, or wait till the server has a restart.
I've got it designed already, mentally. The only problem is getting it to look that way on paper/computer screen. "Circles are nice" "Dafuq is this oval??"
1. You should have armor on. 2. Go in there and fight them with a diamond sword, only 3 can smother you at once and 3 zombies don't cause much harm if you can keep them all back one at a time.