From my testing, I used a SanDisk Ultra C10 card, when writing the image from my mac, it shows the speed is around 15 MB/s. However, I boot the operating system from it and run the speed test (using if=/dev/zero of=/tmp/test1.img bs=100M count=1 oflag=dsync), the speed is only 10 MB/s. This shows that using the card slot may have the limit with the write speed to only 10 MB/s, whereas I do the same test with eMMC and get the result of 66MB/s, so I would say maybe it is not worth using the SD card to run your system. Feel free to post any test result, we can further discuss this.