There are similar contents on Aerosoft forum as well as ActiveSky Forum.
Both threads pointed the problem to ActiveSkyP3D, which is still in beta.
Me personally also had this problem serveral days ago, what I did is to re-install ASP3D
Have done a 1-hour flight, and all seems ok, no performance loss, normally I will get constant shutters and thus fps drop to 20-ish fps after 40 minutes or so.
My spec: R7 3700X\RTX2070, Windows10 Home 2004 , P3Dv5 HF1,ActiveSky B7459
All of these tests are done with Aerosoft A330.