Rendering 5k video (4992x3744) is MUCH slower than 4k (3840x2160) so I tried some tests, all on 30 second clips cropped to 1080. All files are on the NVME SSD and the Video GPU was set to the AMD.
All these numbers are approximate because the usages bounced all over the place although I did run the tests 5 times to get a better idea of what was going on.
First, just playing the clips on the timeline at 1080 gave me about .8 fps on the 5K and about 26 fps on the 4K at Best Full. Interestingly, on the 5K clips, almost all the work was done on the Intel HD GPU 40% utilization vs about 6% on the AMD. However, this reversed on the 4K clips, the HD went down to about 20% and the AMD up to about 40%.
Similarly, VCE rendering the 5K took 1:42 with about 70% of the HD used, 6% of the AMD and 40% of the CPU. Rendering the 4K clip the same way took 0:25 with 35% of the HD used, 50% of the AMD and 70% of the CPU. In both cases, the AMD video memory was barely used at all, about 500 MB out of 16 GB.
I tried disabling the HD Gpu in the device manager and the 5K rendering was slightly faster 1:31 with the AMD usage about the same, but the CPU dropped to under 50%. The 4K render was the same time 0:25 with the same GPU and CPU usage. For rendering, it seems that the HD GPU is mostly getting in the way. For timeline playback though it's essential.
My guess is that a couple of things are going on. First, as has been discussed, the AMD decode hasn't been released, I can't wait. But, I think it's more than that, The fact that so little of the video memory is in use seems suspicious. There's also something odd with the decoding since the the HD GPU reports heavy use of the 3D processing, but nothing for the video decoding hardware. The AMD reports heavy use of the video encoding and a few % for the 3D.
I'm hoping that an upcoming release will make more use of the AMD hardware, it's not even getting warm now.
NOTE: for anyone playing with this kind of thing, Vegas does something in the background when it first starts that eats a huge % of the CPU. Sometimes that lasts for a few minutes and really messes up the timing measurements. Keep watching the task manager till the CPU for Vegas goes to 0%.