I am using class 10 memory card still all apps takes too much time to load... Even tiles take too much time to refresh, app store, apps everything... Nothing is smooth... Apps on android is more smoother than Wp8.1 but why can't wp8.1?
Cards are rated at their maximum speed rating, you never read about their random access times.
WP phones need good random access times, and you generally cant find the stats anywhere.
One of the original WP7 handsets had an SD card in it, and there was a big thread about which cards were good or bad.
Even I am suffering from the same problem.i have class 6 memory card on Lumia 520..i hope the firmware update is our soon