Once again, the X-Plane 11 Beta, as many times mentioned, is considered to be buggy. Laminar Research is likely prioritizing issues with the BASE package of X-Plane 11 before even considering bugs relating to add-ons, such as the Rotate MD-80. If you believe their is a compatibility issue for the Rotate MD-80 with X-Plane 11, you will have to contact the MD-80 developer, not Laminar Research.
As for the frame lag, you are not the first to report that. Laminar Research stated that lagginess was a fair possibility in the Beta, so sit tight until more things are worked out.
But for future reference, bear in mind that X-Plane 11 also brings about more graphical improvements and modified techniques, so "lower frames" compared to X-Plane 10 was generally an expected thing.