Hello, the speed depends on clocks selected for the SDcard. And the mode of the card and data width. Max speed in DS is 133MHz. If you use 3.3. Then max speed is 50MHz. If you have voltage shifter you can run up to 133MHz on SDcard and use DDR mode. This will be the data transfer speed. But be careful about the SDcard operation state machine when after each completed write the memory switch into busy state which can be from 250-500ms. This is defined in SD card specification.
