I'm guessing you're getting the Code Analyzer warning "Calling AXES(h) without an output in a loop can be very slow. Include 'h' in plot function arguments instead."
Unfortunately, image doesn't accept an axes handle as a first argument, so this warning isn't particularly useful in this case. Because of that, it's possible that the guy who made the video disabled that warning. (It's also possible he has an older version of MATLAB.)
To suppress warnings, you can add %#ok at the end of the line, or to disable that warning altogether, go to the Preferences and select Code Analyzer from the left-hand pane. In the bottom pane on the right you will see all the possible messages, and their status. Scroll to the category "Potential Performance Problems". Deselect the third and/or fourth options, as you choose.