I think it depends on the size of the swamp. For example, a small-ish swamp island near our home base was pretty easy to clear, but venturing into a huge swamp peninsula was quite a lot more difficult. That swamp peninsula spawned an abomination once every 3-5 minutes. When we went through the portal our other players built, and went out the castle gates, an abomination rose up suddenly. After we took that one down, another two popped up at a ruined tower we hid in, then another two popped up consecutively as we made our way to an unopened crypt.

Mind you, we're at the Iron age and not fully upgraded.