2. Not using a proper codec. I tried editing some of the 4k hq and 4k 120fps footage that's out there and my 10k PC setup -with 32gb of video ram- couldn't handle them (in premiere or resolve.). I suspect the h265 is the culprit. Why wouldn't canon use braw or prores?
I'd hate to think how big the filesizes would be if they used prores instead of the very efficient h265. You'd get a few seconds of 8K on a card instead of minutes.
You probably just need a video card with hardware h265 decoding.