I am annoyed by fake progress bars.
I do not ever want to see a progress bar rapidly advance to 90% and then slow to a crawl unless the operation really has progressed and then slowed to a crawl.
If you do not know how much of the operation has been completed and how much remains, then show something other than a f**king progress bar. 
And yes, #Mastodon image viewer, I'm talking about you. 
That is all. Carry on.