I've found --some-- kind of workaround. First I went through and Pano-mapped the entire video, jitter and all.
(Now, I have another thread here, where the stabiliser is just atrocious performance-wise in Resolve. So usually I do the Spherical Stabiliser in stand-alone Fusion because there I can get 90 frames per second using the proxy files, then I copy the SS node to Resolve. But I started doing my pano-map in Resolve so I'm kinda stuck there for now. If I managed to figure out proper FUsion transitions -within a clip- I might end up doing the whole thing in Fusion incl. Panomap & SS.. but ok for now, this:)
My video is 22mins long. After about 3000 frames, SS slows to about 1.2s/frame (this is on a 5.7K source, Fusion ignores the proxy it seems.
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So I've pano-mapped the whole 22min video. Since the first ~2000 frames *do* keep a decent speed, I'm able to simply chop the clip up into 20 individual pieces and run a SS node on each of the 5.7K clips within Resolve. It still drops the speed from 10fps to about 4fps by the end.. but that's enough to complete the 1600-2000 frames. THis way I can process the whole clip., eventually.
Would be much nicer to just set it loose & run the whole video ( so I can go to sleep :/ ). But this is manageable I guess. Will still mean a 16hr delay since I now have to leave it and pick it up after work tomorrow, so that's not ideal, but at least it's a workable way forward.
FYI, it's the 5950x/64gb/3090 I'm working with, so specs are unlikely to be the cause of the issue.