The Samsung and SanDisk class 10 cards still have good small file performance, some of the other brands have optimized their larger file performance at the expense of the smaller file performance.
You might want to consider the Koreader alternative reader program, especially for PDF and CBR reading.
An install guide. With a large library you might also want to hide your book files from Kobo's processing, (discussed in the guide).
Luck;
Ken