I'm not sure why they are going with UHS-II. That's 312 MB/sec (bus speed, not write speed). The files are about 25 MB each. 25 MB * 20 fps = 500 MB/sec. How big of a buffer do you get and how long will it take to clear? Now, at the 12 fps rate, it's not so bad (300 MB/sec).
CFExpress is...