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.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?
You probably just need a video card with hardware h265 decoding.