This is something I've seen on a friend's computer, though I haven't experienced myself. He had 100+ fps, held a bow, it went to 20, then he drew it, and they dropped to the low teens. after destroying the bow (it was a TC shortbow), his FPS want back to 100+. This was a while ago, and on a different modpack, though, so I doubt there's anything the modpack makers can do (It's probably something with the mod).
Edit 5: Removed unnecessary text. The super-excessive lag was caused by a mother spider super-cell somewhere under the house. (I live in the tropics). If your sever is having frames issues, warn everyone, then set it to peaceful for like 10 seconds, and you're probably good to go. They can get really laggy once the baby spider count exceeds a few thousand. (Not kidding, I once wore through two diamond swords while fighting my way through a horde of them along with another player, who wore through several weapons of his own. These things are not to be taken lightly )