The portal will drop my FPS from 300+ in a superflat world to 3 or less in a heartbeat. This only happens when I look at it though. Once I look away, my FPS goes back to normal.

EDIT: So it seems as though the problem is the shaders. I removed it and the FPS is normal again. However, it's a great shame since shaders is such a nice thing to have. I'd rather not disable it if I don't have to.

The current version of Iris is incompatible and the next version of Iris will be compatible. You can use https://github.com/qouteall/Iris/releases/tag/ipc1.1.1_1 for now