By way of further explanation: SD cards are optimised for doing very large block writes. A digital camera captures several hundred Mbytes of data and needs to write it to the SD card rapidly in order to be able to take another shot. So modern SD cards have large RAM buffers to accept the data, then after it has all been received they get on with writing it to flash memory and moving blocks of data around to implement wear levelling. Unfortunately we only have 128Kb RAM total available on the Duet 2 series, so we have to do much smaller block writes. You can use the M122 P104 function (https://duet3d.dozuki.com/Wiki/Gcode#Section_M122_Diagnose) to measure the SD card write speed achieved by the Duet.
