Offline, game runs smooth on all stages. But online, if I play on any other stage but battlefield, it'll lag for me. Maybe it's computer, but i shouldn't need a super beast computer to run this game smoothly right?

Lag depends on your internet connection and how far away you are from the other player, as well as their connection. If this is a continuous thing, it's probably your own internet, or you've just got unlucky.

You could probably tone down the lag by adjusting your quality settings.