To answer your question, everything in the game leeches resources, either CPU or GPU cycles, but the order of leeching from largest to smallest on airfields is:
1. Number of airfield trees (this is the second time OBD has optimized the airfield trees because they were so much of a CPU hit when they were first created for the game)
2. Number if aircraft (both at the hangers and on the flight line)
3. Number of buildings
4. Number of men and vehicles
The last (#4) is so small, that unless you have a weak CPU/GPU, you will never notice it because the first two are the biggest leeches.
Try it by picking the same QC (not campaign because of all the background variables), with the same airfield, planes and weather and exact same view. The check the FPS without the mod, and then with the mod and see the difference in FPS between the two on your PC and you will have your answer.