I had the same issue and even it happens most of the time, all you have to find out is it just Apple stores or other services are also choking, fo that do some browsing, check ur facebook videos, try speedtest.net, if other services are working at required speed then its a Apple App store servers problem, which gets better after sometime, and if its overall then do one thing, do the hard reset, means press home and sleep/wake button togeather and wait till iphone restarts by it self and Apple logo showup, just with that leave both buttons, i hope this will resolve the problem, but before doing that check the overall wifi performance.