iOS turn off Fade-in effects?
How to remove the animation when closing an app

Please note: I tested this with an iPhone 5s running iOS 10.2.1. It should work with the iPhone 6 / iPhone 6 Plus as well, but not with newer devices (iPhone SE/6s/6s Plus/7/7 Plus). Also, the animations will be back as soon as you reboot your device.

Part 1/2: Prepare


Open this picture in Safari.
Press the "Share" button on the bottom and tap "Save Image".
Go to Settings > Wallpaper > Choose a New Wallpaper > All Photos.
Select the new image (black pixel).
Tap "Set" > "Set Both".
Tap "Choose" > "Wallpaper" to go back.




Part 2/2: Crash

Now you need to crash the process that's responsible for the smooth animations. You need to repeat the process below about 10-14 times.


Tap on the lock screen preview (on the left).
Tap "Set"
Tap on the home screen preview (on the right).
Tap "Set".


You can verify that animations were disabled by pressing the home button. There will be no animation at all.

If you still the usual animation, go back to the wallpaper settings and repeat part 2 a few more times.

.

Please let me know if this works for your iPhone 5s or iPhone 6 / iPhone 6 Plus.