I did see a video where someone with an RTX 2070 play back 8k red raw fine but that was on windows. (
I'm running Davinci Resolve Studio 17.4.6 Build 4
Red raw has GPU hardware acceleration. Canon Raw does not.
You will also see BRAW is faster than Canon raw as Braw is partially debayered in camera to avoid the Red Patent.
Use 1080p, lowbitrate optimized media for timeline edits where you need speed. Switch back to raw files for color grade where you need accuracy.
I'm did do a 4k optimized media, but I'm not editing them, just coloring it. (it took like 6+ hours or so.) I shot in clog3 also. So I turned off the optimized media and am now just trying to color it.
I know the OP said it's not the drive but it may be overheating causing throttling. The OP doesn't mention any other drives. If the media is on the only drive anything else accessing the drive might cause the slowdown.