oh that, very frustrating same here, today and yesterday, one gym nearly impossible to prestige, I get 1 pokemon in, and it grinds to a complete halt.. trying to load all the sprites etc, I had to move position to actually fix it.. still slow, but at least I could get the team together.
today, that happened.
30-40 minutes later, GPS signal not found.... then it finds it again.... 3 minutes later have to completely restart as the GPS goes kaput (avatar just stands in 1 spot) and that stops any apps opening at all, so have to hard reset. (iphone hold both buttons down) then exactly the same 5 minutes later, I exit the game, try to load another app. again nothing.
It had been fine all day, I had been in a large park, no loss of signals no lags no nothing...