Whats a good card ? Difficult to say as the speed/IOPS depend on the combination of you e-reader and the sd card itself. Cards performing well in a PC, camera or raspberry pi do not have to perform well in the e-reader. As you can see here:
https://www.mobileread.com/forums/sh...d.php?t=315976
If you want performance, you pay or you take a gamble with multiple cards.