On the iPhone you need to take it out of your pocket, wake up the screen by tapping it once, or lifting it to some angle that the screen turns on. Then you need to long press the on screen camera button.
If you enabled the double tap, you still need to take it out of your pocket and wake up the screen before you can double tap.
On Android you could have the camera ready before the phone is even out of your pocket.
Also, from an awakened lock screen, as an alternate to the long press of the camera button, you can just swipe left on the screen, revealing the ready camera app, which saves a second or two.
On the iPhone you need to take it out of your pocket, wake up the screen by tapping it once, or lifting it to some angle that the screen turns on. Then you need to long press the on screen camera button.
If you enabled the double tap, you still need to take it out of your pocket and wake up the screen before you can double tap.
On Android you could have the camera ready before the phone is even out of your pocket.