As an Iphone user, in my imagination that people use phone mainly when they try to pass the time and so they would use the certain app, and check the phone when they see the notifications, and use the home button with their right thumb a lot to change the position of apps, and to return to the main page in order to switch to another app.
Unlike myself, some users do use left hand when they are using the phone, according to one of the user I observed, he is a right hand user for most of the time, however he felt more comfortable when using left hand with the Sony phone he had. Swiping down the menu is also really commonly used and cause no time delay whatsoever.
Turn the phone 90 degree before using the camera, it takes a moment to open the camera, which cause a time delay. It took a longer time when opening the app store , and reorganizing the arrangement of the layout of the apps. The longest time goes to when try to enter a number during the call, it takes a while to move the phone away from the ear, see if the screen is light up again, if so, switch to the dial mode, and then input the number.
According to what I’ve read, I believe that the best design should be the one that requires least thinking process. But mobile phone is a such commonly used tool that even people like us rarely notice anything or feel anything as for the progress while using it, that’s what makes observation so important.