Game lags in the final few rounds because there are too many units and towns on the map. Make sure have a good amount of ram before you play. No problem in the first ten rounds. Good game overall. I wonder why I can't play "alien" race ;)
This is tip for all of you having troubles. You need to deploy a lot of cheap units (rifleman and assault units) fast for missions without tanks. (more units will provide you with morale boost)