Android zooming (Magnification gestures)
This is a very useful, yet not widely known, feature. This way you can zoom anything on your display simply by three taps. To try it, switch Settings -> Accessibility -> Magnification gestures to On. Triple tap and ... voila ... you are zoomed! You can zoom even more by pinch gesture like with any images in gallery. Move zoomed surface with sliding your finger around the display. Use another triple tap to disable the magnification.
Why is my phone laggy with Magnification gestures on?
It’s because of a timing problem. When you tap on display and Magnification gestures are on, the system has to wait for a while if a second tap comes.If it does, the system has to wait for a next one. This is why your taps and double taps respond a little bit slower with the zooming enabled. So you don't have to worry about your battery, it has no impact on it. It is not slow because of high power consumption but because a wrong choice of the gesture. I have no idea why it’s not a pinch gesture, where this problem doesn't exist.