Lock Screen Issue

I have an iPhone 64 GB and last week, I updated to iOS 9.1 from iOS 8.xx. After the update, my phone sometime lags and the screen didn't move even I scrolled everywhere. And the problem is, when you locked your phone with assistive touch, the screen it should be showed black blank screen and locked instantly, but with my phone, sometime when I locked my phone, it will shows the black blank screen and then the wallpaper appear for a while, and then the phone locked. Anyone could help this issue? Is it a bug or something?