Yes, except for 8K, the UHS II slot is very fast, faster than needed for 4K, for stills, its much faster than my card reader. I have two 150 MB/s V60's for my EOS R. I can't measure the speed until I get a new reader, I'll do that when I get a CF express reader. Of course, I do not play video from my V60 card, just download it. Who plays native unedited videos from a card? I download my files to my computer for editing.
For my R, note that a class 60 is the fastest needed for 4K. A class 90 is probably needed for R5 8K, but I'd get a CF express for that. I have no plans to use 8K, or 4K for that matter.
I would not run out and buy cards in advance, we may find that some are incompatible, and your return window would be past.
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