Frozen, Slow Touchscreen or Keyboard
-> Solutions :
General solution is to perform a soft reset by holding down the Home button and the Sleep/Wake button until the device restarts and you see the Apple logo. Or double tap the Home button and make sure you swipe to close anything you don't need running. Here're some specific solutions:
> Frozen or Unresponsive Screen: If your phone has completely locked up, press and hold the power button and Home key at the same time until you see the Apple logo. This will reboot your device.
> Slow Touchscreen: Devices typically begin to slow down as their internal storage fills up. If you have a lot of unused apps or old photos, move them to the computer for backup and then delete them.
Moreover, you'd better disable the parallax wallpaper. On the iPhone 4 and 4s, go to Setting > General > Accessibility > Turn on the Reduce Motion option. Newer iPad models, along with iPhone 5, 5s, 5c, go to the Wallpaper & Brightness settings > Tap the lock screen wallpaper > Toggle Perspective Zoom to off.
> Slow Keyboard: iOS 8 brings the biggest changes to the keyboard, making it predictive in its text selections and choosing autocorrect substitutions. And also for the first time, adds support for full keyboard customization, including installing 3rd-party keyboards like Swype. Go to Settings > General > Reset and tap Reset Keyboard Dictionary.