Ok, so this is pretty pants. Decided to aggro my first settlement to go grab some data and materials. I've only just started doing Odyssey stuff, but so far performance has been more or less ok. Good framerates generally with my new card, with the odd stutter/dip for a few seconds while entering bases and running around, but it quickly settles down.
However, after aggroing a station from above (missile spam) frames started tanking. CPU and graphics card not even breaking a sweat, about 30%.
After bombing the hell out of the base, landed out of air defense range and switched to SRV for the approach, and this is where it got really bad. Over the rim of a crater, so couldn't even see the base, yet it kept dropping to 1 FPS for extended periods. Basically my approach the whole way, 2.5km, was stuttering along painfully at low FPS. I couldn't drive fast because the SRV was uncontrollable.
Finally made it to the base, and things started to get better, still not great, but almost playable. Killed off a few NPCs and things got a bit better. Then the dropships came in, dropping NPCs, but by that point the stuttering had gone.
After taking care of the dropship waves, all good. No performance issues at all.
Throughout the whole time, not once did my CPU or GPU show any signs of stressing. They mainly trundled along at around 30%... highest were peaks of about 50%.
Still something fundamentally screwy going on.