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Yes, indeed the AMD Radeon Pro 5700 is at the top of that list in the link.
As an experiment I decided to checkout 8k video editing on PD14 as it was done 6 years ago in the link supplied. The vp9 encoded 8k 60p and 30p YT videos have audio only in preview. Converted them with HandBrake to h.265 at both 60 fps and at 30 fps at the low bitrate of 21.6 Mbps which is the same as the source. To my surprise these 7680 x 4320 resolution videos can be edited easily in both of my pc with i7 cpu’s without shadow files.
My GoPro hero6 4k 60p h.265 68 Mbps encoded videos can hang in the preview while editing and requires shadow files to be enabled for a good editing experience.
8k Vp9 videos can be edited natively in the later versions of PD like the converted h.265 ones. So why can I edit these 8k YT videos with Nvidia hardware decoding unchecked and without an AMD Nav10 gpu?? … whereas my 4k GoPro hevc 60p videos require shadow files to be enabled. The answer I believe is in the low bitrate. You can perform your own tests and post what you find.
Not sure I see anything unusual with your noted behavior.
But yes, bitrate and codec complexity is what pulls PD timeline fluidity down. Keep in mind resolution is contained in bitrate. The basic math was shown here
https://forum.cyberlink.com/forum/posts/list/18251.page#post_box_93985 In my experience I'd say PD timeline fluidity starts to have some issues even on high end equipment at above say 40Mbps or so for a single track of advanced codec content, so it does not matter if 2K, 4K, 6K, or 8K for the most part as that's already taken into account in the 40Mbps bitrate. But for similar quality playback, higher resolution needs higher bitrate if truly playing back on appropriate equipment so not really proper to scale to same Mbps as a lower resolution.
My guess, you simply can't produce (full encode) to a 8K user defined profile unless you have the hardware to do it, PD CPU encoding won't cut it nor many iGPU's or say a 900 series Nvidia or below or I'm guessing here, below VCN2.0 for AMD.
PD really does not edit a 8K VP9 video natively, PD versions can simply import it but the timeline display is never 8K so you are always editing a proxy display, not native. VP8/9 decode part of Win10 current release.
Jeff