everything you do and make in wc3 takes up memory, picking all units in a region, that stores all those units into memory under a variable, when you kill them, that variable still exists with those units stored in it. Even though you are never going to use that again its still taking up the memory. It may not be much, but repeat this several hundred times, and you run out of memory and start to lag/desync. This is what i understand memory leaks to be.