ubidefeo
The above R4 result use array transfer to get a big improvement over single byte.
It is still very slow for 24 MHz SPI. I would expect SdFat to deliver about 2,500 KB/sec. I get about 430 KB/sec.
I implemented an array transfer for SAMD21 that gets 1360 KB/sec at 12 MHz. This is about three time the Arduino Core SAMD21 result.
Here is a link to that mod: