I've noticed a consistent pattern of the game slowing down when playing on the same map for a long enough time. I've never seen it on maps with a finite number of enemies, but maps with infinite respawns like Lost Valley in BSM do seem to have the problem. Here's what happens.

Everything goes fine for about 10 to 15 minutes on the map. Then I'll start to see the frame rate (i.e. rate of screen refresh) steadily start to drop. Over the next 3-5 minutes, the frame rate gets down so low that it's like seeing only a few still shots, making it very choppy and hard to move around. If I die though (which is easy to do with everything so bogged down) the problem will be gone when I revive.

This is a very repeatable problem and not due to internet connection or other traditional lag sources. Most times the game will be reporting a good ping (sub 200) even at its most choppy, and there are none of the symptoms of "normal" lag like mobs not moving, other players teleporting around, etc. It's pretty clear that there's no problem contacting the server, just something local going on in the client itself.

My best guess is that there's some kind of resource leak happening with each mob or skill use or something. On most maps it's not a problem anyone sees because the place runs out of targets before the leak slows anything down. Also, it's definitely a local problem because dying clears it right up. I think the map goes through some kind of local reinitialization upon dying and reviving, since you'll see any notifications like a friend request pop back up again after revive (as if you just entered the map for the first time). So I think whatever is happening when a player "joins" the map again after reviving clears up the issue.

I see this problem consistently on my Galaxy S4. It is running Android 4.2. I'm not sure if it happens on other platforms like ipad, but if I get a chance I'll give it a try. If anyone else sees this issue, could you chime in with the device you're using?