@KurtE Made the overlay changes to the q, rebuilt and flashed the q. SDfat lib now runs.
Running the stock SDFat bench sketch from the IDE at 24 Mhz shows pretty lousy performance as expected:
Type is exFAT
Card size: 30.98 GB (GB = 1E9 bytes)
Manufacturer ID: 0X27
OEM ID: PH
Product: SD32G
Revision: 3.0
Serial number: 0X6C06C24C
Manufacturing date: 8/2021
FILE_SIZE_MB = 5
BUF_SIZE = 512 bytes
Starting write test, please wait.
write speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
50.89,14220,10050,10055
50.68,10079,10050,10069
Starting read test, please wait.
read speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
51.30,10142,9970,9978
51.34,10028,9970,9971
Done
Type any character to start
You already pointed out the reason - large gaps between bytes/words which was a know issue in zephyr but not sure if they ever incorporated a fix for that.