Played with both mods up until early hardmode, when Cryogen and Aquatic Scourge decided to drop our FPS to the floor, very jarringly.

We noticed that their Infernum specific projectiles have these light borders, to help with visibility. Our theory is that they also produce some light, which your mod seems to render in an unoptimized fashion when produced at amounts never before seen, like atleast 50 projectiles an attack. I don't see the same fps drops in singleplayer though, which might mean it also has to do with general poor performance of multiplayer modded. But I am sure that it is your mod, as without it the fight no longer had the fps drops.

So, to reproduce, in multiplayer, fight either Cryogen or the Aquatic Scourge in Infernum difficulty and observe the fps drops when they use their projectile spam attacks.