Well, this is what we do all the time, if we are not running those fancy app testing tools. In an open device lab or a private one, we use stop watches and perform certain actions to check if it loads fast. Or, you may even go ahead and say that you mentally calculate the time if you do not have access to a stop watch on your phone (which is next to impossible). The bottom-line is, this is one of the questions that is asked for the sake of asking. You must stand your ground and not be overcome with nervousness or fear.