I've been doing similar, although reached a slightly different conclusion. It seems to me that the menus (even idling in them) use full GPU. I'm not convinced it's necessarily rendering stuff (and if it was, it really ought to cache it after the first time!) - I think this is just the result of a really bad performance UI they got away with launching.

I don't seem to max anything out when flying - even GPU has a little headroom according to task manager.

Hopefully Asobo will spend time on performance, rather than just bare functionality, as there's definitely lots of scope to fix things like frame pacing and overall performance.

I was watching my task manager as I played. On my 4k @60fps on the Very High graphics settings and the GPU was spiked at 100% the whole time (GTX 1660). Sounded like the propellor was in my computer room. On my 1440p monitor are @75fps I was at Extreme graphics settings and GPU was at 40%.

I also been running the task manager, mostly to check when it seemed to hang it was using 100% of the transfer with the hdd, didn't look at gpu usage though, will have a look the next time.