> Progress bars have very low credibility on Windows, because users have learned that they're basically useless as an indicator of wait time. A progress bar might get stuck at 7%, then suddenly rush to 100%; conversely, it might get stuck at 95% but never finish. The bar offers no real indication of the actual level of progress
I disagree with this; I find the progress bars more credible with erratic timing. (And ideally, a display of the task currently at hand, like "Copying tiny file. Copying tiny file. Copying giant file............")
A progress bar that smoothly fills from 0 to 100 looks like an animation that somebody thought it would make you happy to watch. A progress bar that lags at 7% and then rushes the rest of the way looks like the software has some internal metric for task completion, and is reporting according to that metric. This implies that when the number changes, progress has happened, which isn't the case for a progress bar that isn't affected by workload.
The software can't use "how much time has elapsed?" as a progress metric, because it doesn't know how much time things will take, and because the passage of time does not actually cause -- or reflect -- any progress. That progress bar would be a spinner, not a progress bar.