[Fargren]

I'm pretty sure in most systems I use, processing time is not deterministic and it depends on what else the system is doing at the time and it's temperature, among other factor of which the progress bar won't be aware.

A progress bar will usually present the "expected" duration of several operations, [almost] none of which have a deterministic duration, or a deterministic ratio from one to the other. The best we can do is put heuristics in place to estimate how long the operations will take. We have some choices, such as estimating the total time or estimating each sub-operation separately. We can even use AI for the estimation, if we want to get fancy. But the problem is not tractable, and we'd rather given estimates where we can that just use spinners everywhere.