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Comics are hosted on a USB Drive ( My Passport) through a powered USB hub.
I've tried with files on Pi memory card, same issue.

I've checked with top, one or two cores are being used mainly up to 100%.

My previous device was EEEPC 1005HA ( Intel Atom N280 1.66GHz) running xubuntu 12.04 with 2GB of RAM. Comics were hosted on same USB drive.
Performance was indeed fine even for big pages.

I've installed mcomix just to check and big pages are loading fine.