We used to have Twilight Forrest, but unfortunately it caused more lag than even Mystcraft. After so many crashes and having to edit people's files to get them back to overworld, we finally did away with it. Since we've gotten rid of it, we haven't had any major issues with lag (other than people traveling at like mach 5 lol). I kinda liked that mod too, but for some reason it was constantly having corrupted chunks, which is an issue that hasn't happened again since getting rid of it.
But yeah, before we did away with it we actually took a vote, it was unanimous to remove it. (as I said before, this modpack is actually born from a server that a group is playing on, that's how we decided what to add and remove). The next update is going to be centered mainly around magic, you play with magic mods much?