1. I'm playing at 4K not 1440 as Morbad, so it is less CPU constricted... also, how much is "significant upgrade"?;@Morbad would disagree with your statement - the one i put emphasis on (bold/yellow)
Oh, Morbad already pointed out that a cpu upgrade prove to be a significant upgrade
You are definitely holding back that RTX3090 with that CPU - at least in Odyssey
2. I have to find a very specific hight/point to be able to replicate the low FPS whilst hovering at an empty settlement because it seems to be triggering some constant rendering and redraw refreshing, hence the low FPS with CPU temperature spikes (this is why it seems AVX is at play here)... also, I'm reporting the worst cases I've found... normally my on-foot CZ is at 65FPS... but it sometimes briefly drops to a minimum around 40s in some cases, specially in the dark.
3. Now the main point... after Frontier actually fix this mess, the OP might not even need to upgrade his hardware... and if he does think he still needs it, RTX4000 series would be already released, new AMD/Intel CPUs... it would be more than worth to wait another 6-12 months for it.
Bonus point: As performance differs from place to place and a lot of conditions are at play, I was inviting @Rat Catcher to join me on foot somewhere so we can try to figure out how much difference his AMD CPU makes on eating macarronic code... for science!