So the title would more accurately be "UI responsiveness matters"? I doubt anyone would think this kind of lag doesn't matter. It sounds like the software was designed (and tested) for better hardware than it is running on, or at least different hardware. But that's just a guess.
Let's say you have a client app with checkbox that toggles some boolean variable that communicates with a server. If you're on a 3g connection in an ambulance, it might take a few seconds to send that request to the server and get a response back.
An unresponsive UI would sit there and make you wait for the request/response to complete. You click the checkbox, then nothing happens, then suddenly, some time later, the checkbox is checked and the page refreshes, or a state change happens. It's jarring and weird. A responsive UI would update the state of the app based on the user's actions even if the request hasn't completed yet.
Tightly related to this the concept of optimistic UIs - the UI acts optimistically and updates the state of the app under the assumption that server-backed state changes will work. The iPhone's messages app sending messages is the prototypical optimistic UI interaction. You send a message, the blue / green speech bubble shows up on your messages app while the message is in flight. If it succeeds, nothing changes and you're none the wiser. If it fails, you get an option to delete the message or resend.
A responsive UI is necessary, but it's not sufficient to make up for a slow back end.
Performance does work as fast as possible regardless. You always want more but you’re capped by the system, hardware, and algorithms and not human perception.
Feels like I’m splitting hairs though
Even when responsiveness is great, you can still work on performance to improve: resources utilisation, energy usage, jitter of responsiveness, etc. Users may not notice, but the spending account may do. This distinction is more relevant with multi-user or time sharing systems, less so with single-user game consoles. (If you're hitting the right framerate at the most demanding scenes, why bother?)
That the task takes an hour to complete may or may not indicate a problem with performance.
The fact that the task shows a progress bar at all is likely because at some point the task would complete synchronously and without a progress bar in say 10 seconds or 60 minutes. In that case, the programmer mistake was to allow the problematic performance of the program affect the responsiveness of the program.
So a progress bar is used when the performance can't be easily fixed but the program must be responsive.
I know it's easy to dump on electron, so I will. Multiple electron apps, especially the ethereum client, are the worst programs I have ever used, because there is enormous lag on a computer that runs everything else flawlessly. It's so bad it can't even be all blamed on electron, but more than bloat or memory use, interactivity is what I consider the most important factor in an interface. I even put it above great design or intuitiveness. I would take blender's ridiculous interface choices over something better that lags like the ethereum client any day of the week.
EDIT: Or to put it another way: "perceived performance can be more important than actual performance".