Those that are predisposed to build slow sites despite all the evidence about how detrimental it is don't really need any excuses for some reason.

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.

I think that if I ever had to load your "fast" website on a dial-up connection I would hope that it had some sort of progress bar to fall back on.