After playing a few hours, FPS drops from 40 to 5FPS

As addition to the test I posted here before.

I just started the second 5h flight with the sr22.
On the first leg i had about 45fps at the destination airport. Sadly couldn’t land, CTD.

Now I restarted and respawned at that airport.
Started flying with 55-60fps.

Now after 1h my fps are still the same.

I didn’t change anything from the first flight (settings)

Now my
CPU’s at 28%
GPU at 97%
VRAM at 33%
RAM 18%

The only thing now is, the sky is almost clear, before I had some broken clouds.

But why does my GPU ran at 97-100% in clear skys and is going down to 86% with clouds and bad weather.
Shouldn’t that be vice versa?