Kobo Touch 2.5.2 takes a really long time to boot with epubs on the microSD card.
If I boot without the microSD, it looks like it's stuck. After a good long wait (~30mins), the covers show up - along with a good drop in battery %.
It boots up real quick if you booting without the microsd card. And no, there isn't anything wrong with the microsd card either, it works just fine on my wife's Kobo with 2.4 firmware.
Reverting to 2.4 seems to be the only solution. More in this thread:
https://www.mobileread.com/forums/sho...d.php?t=214455