UPDATE Dec 2021: the OWC Atlas S Pro SDXC cards are top notch, and even a little faster than the Sony TOUGH SF-G SDXC cards. I am switching to them. See TESTED: 256GB OWC Atlas S Pro SDXC Card (plus comparison to Sony TOUGH SF-G). CFExpress is even faster, but 2X more expensive.
CFExpress is claimed to be incredibly fast. Sony claims the following for its insanely expensive Sony CFExpress Type A cards:
Read Speed: 800 MB/s Maximum (Burst)
Write Speed: 700 MB/s Maximum (Burst)
The qualifier there is “burst”.
For the test below, 1000 files were written, then read back, with the timing for each graphed, as shown below. Each file being 78.6MB, that’s right in line with a compressed Sony A1 raw file.
The Sony CFExpress card can maintain burst write speeds of ~650MB/sec for about 110 files, which equates to 5+ seconds at 20 fps. That ought to satisfy most photographers most of the time.
But keep shooting continually, and write speed plummets to as low as 65MB/sec, oscillating mostly in the 90 to 180 MB/sec range once it is really pushed hard.
Average write speed: 291 MB/sec <=== WAY below burst speed
Average read speed: 785 MB/sec
Less clear is whether the card can “recover” from repeated bursts of ~110 files without breaking down in speed as seen (this was a continuous unrelenting test). Thermal throttling might be at work here.
Read speed matters for playback and downloading the card. While that also drops with sustained usage,the average speed is fully satisfactory.
A much less expensive Sony TOUGH 128GB card can be had for a slightly lower price, and is rated at 300 MB/sec reads and writes, with a minimum writes speed of 90MB/sec. And without the hassle of buying a Type A card reader—and some Macs have an SD slot built-in.