I have noticed the same kind of behaviour too. Not to quite that degree, but there is certainly a performance loss as time goes by.
I’ll often start a session getting a comfortable 60 FPS with modest CPU load, but after a few legs on a bush trip frame rates will stuck in the mid 50’s, and the effective CPU load has more than doubled. This is despite my destination on these flights often being quiter/less dense than the departure, which should result in better performance, not worse.
The game also has bizzare GPU usage. I can often see the GPU load bouncing around by around 25% and the graph in monitoing tools is all over the place. MSFS will claim ‘limited by GPU’ the whole time, but the GPU itself very much disagrees with that.
It’s quite clear there are some significant performance issues around. My general frame rates and overall stability of performance is much, much worse than it was ~6 months ago. The various fixes have largely done nothing to actually help from what I’ve seen.