In my experience, this wouldn't be needed if the rest of the app ran at native speed. There would already be a natural delay that would be noticed by the user.

I once did a progress dialog for a Windows app. It had a delay so it wouldn't even show until 0.5 seconds had elapsed - if the operation completed in that time, you never saw the pop-up. Once the dialog appeared it would stay on screen for at least a second so you wouldn't get freaked out by a sudden flash, even if the operation completed immediately after the 0.5 second delay.