Water and lava flows can look cool, but the physics calculations they require hog memory. I don't know for sure if the memory they take up is a large portion of the total, but some players have really gone overboard. Asking builders to show a little restraint in placing flows might help the lag situation.
Still water/lava blocks (like those in the lakes) shouldn't cause problems.
Still water/lava blocks (like those in the lakes) shouldn't cause problems.