Noticed:
To all reviewers:
To fix the issue with lag, simply exit the game and come back and it works for awhile. There is an auto-save feature, you know. It only takes a second to get back in.
To anyone having a difficult time with all of the levels, saying that they're impossible or what-have-you. I literally just beat the game. Just have a little bit of patience, keep upgrading your guys. Speed, lifespan and the area of initial attack are the determining factors in the early areas.
The way I did it.. I couldn't tell you the order in which I upgraded my zed. But! I can say that I destroyed every city in every country in this order: Africa, Middle East, India, China, Indo-China, Philippines, Japan, Australia, Russia, Europe, South America, Central America, United States, Canada, Greenland.
It's really simple, if you find yourself having difficulty ending a city somewhere, just move on take out some of the smaller areas, upgrade a bit, then return. By the end of the game I was destroying cities in a matter of seconds (mostly thanks to MJ :P)
Just persevere and you'll find that it's a relatively easy game.