I've already said it before Cris, turn on the flash.

With regards to no flash, if the software is saving an image from an instant before it actually detected you removing your finger from the screen, then that will approach zero lag. If it's not, then as small as the apparent lag may be, it's still not zero. And if like most people, one first pre-focuses/meters by tapping the screen and then presses the shutter button, it's far from zero.

That aside, none of this affects the quality of the images produced by the sensor, so don't think for a second that my initial argument has somehow been subverted.