One thing about swamps that no one seems to mention is that the larger the swamp the LESS dangerous it is. Smaller swamps, especially those tiny coastal swamps seem to have vastly elevated spawn rates. I think what is happening is that the entire swamp area has a spawn rate but when you compress the area vastly it jacks things up.

My first swamp biome had anywhere between 5 and 9 mobs constantly around and respawning. MY second swamp was far far larger and I could go minutes without getting raped.