First off...there is a limitation even in your statement: jpeg to the second card slot. And maybe I am just a jaded former owner of the 5DIII (overall I loved it, but I hated the SD card slot), but I tried jpeg to the SD (UHS I?) card slot but eventually would just turn off that card slot whenever I wanted to truly get all those lightning fast 6 fps.
But, to better respond, I've gone and looked up write speeds to UHS II cards....seems like most of the "fastest" UHS II cards have "up to" 200 to 260 MB/sec write speeds in this
article on fastest cards from B&H. A few quick searches on regular (but still Lexar, Sandisk and still UHS II) and I saw 120 MB/sec and 160/sec go by. (EDIT--just looked at my own SD cards that I have "around"...Sandisk Extreme Pros from a couple years go --95 MB/sec...). As I actually shoot RAW to the CF card and jpegs to my SD card in my 5DIV, I just popped out my SD card and looked at a few of the jpeg file size. The low ISO shots were 5-7 MB and the high ISO shots were 13-15 MB. Scale that up to a larger sensor rumored to being in the R5, I am going with 9 MB/sec low end and 21 MB/sec high end (45/30 x 6 and x 14).
So, 12 fps x 9 MB = 108 MB/sec, 21 MB/sec x 12 fps = 252 MB/sec. But then, jump to the 20 fps and you get 180 MB/sec and 420 MB/sec. Bottom line, some of these will require buffering, even with jpegs. This isn't the end of the world. Actually, fast cards paired with even a moderate buffer are still going to be pretty impressive and just fine for most applications.
But, there is a reason the 1DX III has dual CF Express card slots. This is not that.