If your iPhone has suddenly become slow in browsing Internet, taking photos, browsing in camera roll, or with apps in general, try to clear the iPhone app store cache.
Navigate to the App Store on your iPhone
At the bottom of the main screen of the app store, tap any of the 4 icons at the bottom of the screen 10 times in a row (Featured, Top Charts, Explore, Search, Updates). You will see a blank screen briefly like below. Then, app store home page should appear normally.
Congratulations! You just cleared your AppStore cache! This may have been the first time ever you did this so you might notice a tremendous improvement with your iPhone performance immediately.
See also: iPhone Restore iCloud "Account Details Unavailable"