I strongly think it´s a hit from the mission rather than the map itself. The new 3D model B-1 seems to be heavy on performance, I used one for my regular Nellis template and fps tanked. On this mission, you have two B-1 leaving Kandahar. Plus the two A-10. Plus a squadron of F-14, another of F-4 (both being Heatblur hungry for performance), lucky for you they also placed low poly C-17, otherwise...
You even have a couple of armed humvees running a patrol alongside the perimeter fence.
Im sorry but it seems too much for most systems to handle.
Ala12Rv-Tundra's post in F-14 - Instant Action mission: F-14B-Cold Start (Afghanistan) - Low FPS when initially parked at Kandahar was marked as the answer