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.