If you're having lag, just nuke a bunch of your addons. This happened to me where the map became literally unplayable because I had too many addons and my FPS would drop from 60 to 10 every two seconds. If this doesn't work, I dunno then.

It also might be because of bad computer. I had this problem too, after pc upgrade its gone.
Mainly it propably RAM problem, upgrade your ram if you have small amount of ram memory.