I also noticed this problem, with the F/A-18C only. Yesterday I did some testing.
My basic PC specifications are: Ryzen 5 2600, GTX 1060 6GB, 16GB RAM @ 1080p.
In ME, the only unit on the map is my aircraft...
I load at Creech AFB in the Hornet. ~80FPS is average. I take off, ~90FPS in average. I land, everything OK, but FPS is decreasing as I am losing speed. As I look at the most populated area while stationary the FPS goes as low as 35FPS, ~40-50FPS when looking forward at almost empty landscape. After taking off the framerate gets better, but if I look at the airfield, it drops back to 35FPS.
I load the Tomcat. FPS is around ~10 lower compared to the Hornet, so ~70 while on ground. After I land the FPS is consistent with the starting FPS, ~65FPS when looking at the most populated areas.
Then I loaded the Hornet airborne, just before the runway on final. FPS goes down again, from ~80-90FPS to ~40FPS after landing. I load the Warthog in the same configuration, FPS is above 100 in the air and around 80-90FPS after landing when looking at the most populated areas.
The same thing happens at most airfields as far as I can tell. Before taking off and taxiing the performance is good, after landing the performance is bad. So I wouldn't say it's a shadow issue. Strange, strange...