Use double tap to wake up phone (enable in settings, touch menu) and install Touch and Lock and pin to your start screen to tap and auto lock. I pin mine on the right hand side right near where the power button is located. Easy to tap quickly. Touch and Lock | Windows Phone Apps+Games Store (United States)
There are other touch lock apps but some have a pause or splash screen before locking. Touch and Lock is instantaneous.
There are other touch lock apps but some have a pause or splash screen before locking. Touch and Lock is instantaneous.
No splash screen! Just open the app and pin any shortcut tile you want 
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