I hope progress bars will not be an excuse to build bad slow-loading websites. I think if you are not GMail you should stay away.
In all seriousness, given that there are a lot of use cases where an app might be fetching external resources without clearly informing the user as to (i) what's going on and (ii) the current progress, I'd wager the benefits of providing some sort of indication would be measurable even if it's rather fast. I don't really see the presence of an indicator, in any case, enabling an app to get away with slow/slower results.