For some reason, gloom slimes lag out my game. Whenever they spawn and remain alive, my FPS drop from stable 30, to between 0 and 2. As soon as they die or despawn, the FPS return to normal.
No other enemy does this to my game, only gloom slime. The weirder thing is, that this wasn't happening until the latest update (the one for newest version of tModLoader). Before that, I had no problems with gloom slimes, nor any other enemy.