That still means to produce a RAW file you also need to produce a jpg and now resize it, I think twice as research does suggest there are two jpegs stored in a RAW file, so I can't see how the original supposition, in the article, that writing two RAW files is fast than RAW and jpeg.
Yes, when you parse a .CR3 file there are two JPGs, one is a thumbnail image (tagged THMB) that's 160 pixels wide, another is a preview image (tagged PRVW) that is 1620 pixels wide (sizes for an EOS R .CR3 file). Not really relevant to Cable's post, though.
The statement by him you're referring to is: "
The SD Card slot is slow. I was shooting RAW plus JPEG to both the CFexpress card and the SD card since I couldn’t easily open the R3’s RAW files yet and needed the JPEGs. Some people don’t know this but shooting RAW plus JPEG slows the camera down more than shooting two RAWs because the camera has to process the image twice."
I'm not sure he's correct that the camera has to process the image twice for RAW+JPG (seems inefficient and therefore unlikely), but he is correct that shooting RAW+JPG is slower than RAW alone, if only because the camera has to store both the RAW and the JPG files in buffer and then write both the RAW and the JPG files to the card, instead of just the RAW file.
The way he describes it, he had it set up to write RAW+JPG to Card 1
and RAW+JPG to Card 2. In other words, he selected RAW and JPG (presumably large/fine) under image Quality and has Record Options set to Rec. to Multiple so both formats are written to both cards.
So he's talking about writing RAW+JPG to two cards simultaneously vs writing just RAW to two cards simultaneously (which is the same as comparing write speeds for RAW vs. RAW+JPG) and of course writing just RAW will be faster. In other words, he really means that writing two RAW files is faster than writing two RAW files plus two JPG files.
What I think you're talking about is writing RAW to the CFe card and JPG to the SD card, with the above Rec Options set to Rec. Separately and RAW set for one card, JPG for the other. That would be faster than writing RAW+JPG to the same card, and I think faster than writing RAW to both cards because of the slower speed of the SD slot.
Although RAW to CFe and JPG to SD is faster, there's the (slim) chance that a card failure would mean the loss of one file format, meaning no RAW file for him to play with later if the CFe failed, or there's no way to convert R3 RAWs for now, no way to use the image immediately if the SD card failed. So Cable chose RAW+JPG to both cards as the safer choice, but slower meaning he missed some shots.