I found my "culprit": FlyWithLua (or a script in it: like "XPRealistic Pro")
- Fps went down with >10fps and a lot of stuttering occurred (12-15fps) in the-middle-of-nowehere-SCIP(!)
- Default C172 also suffers under FlyWithLua, but that plane is still "simmable"
- Also note worthy: while flying the TBM900 3jFPS-wizard shows 26 fps, while X-plane's data shows 21 fps at the same time. While flying the C172 both results are equal.
So I removed FlywithLua and took a long shot to re-install all other plugins and let them run. Very good results. Then I left SCIP and cranked up my scenery step-by-step.
More important: I did a three hour VFR-flight hopping from airport to airport and did not experience the problem of this topic: about the continuous low fps after a long flight. The whole tour went great. Only the last 20 minutes I encountered the <20fps-ceiling when approaching Schiphol with a lot of things going on, like World Traffic 3 aircraft coming and going.
So maybe, maybe, this problem has something to do with FlyWithLua.
Hope I contributed with this little research.
All the best.