A progress bar isn't generally hard.

What is hard is making it move smoothly while still only incrementing and getting the correct time. But I don't think there's a single person that still cares about smoothness except for the very extreme cases. By now, everybody has learned they don't work that way.

Anyway, about those extreme cases, just avoid filling the bar up to 99% on the first 10 seconds and leaving it there for the next 2 hours. Just do that and you'll be already better than Microsoft, so nobody will complain.