PMDG - at least for me - does take a hit on frames - I'm doing about 18 FPS on the ground in VC, and mid-20s in VC, in the air: mid 30s to 40s for the 2D.
I'm also running REX, GEX, U.T. as well as several areas of MegaScenery - also FS2Crew and sometimes Electronic Flight Bag. I normally use VC for most of the flight, but - especially if doing a manual landing due to FPS - I switch to 2D so I can get a smooth response.
I've seen others with slightly higher frame rates - 20s on ground and 40s in the air so I'm sure I don't have things set perfectly, but that is what I'm getting.
Wonderful product! Can't recommend it enough.