It's not only the communication between the task and the GUI that is slowing things down (if it is at all, which I doubt). Even more, I think, it's the fact that macOS, the OS in question, has some conventions for windows, controls and so forth, one being that it takes X seconds for the window to appear and X seconds for it to disappear. If it was being shown and hidden again as fast as the task starts and finishes, it would be just be a flicker on the screen, which would confuse the user. It also doesn't make sense to spend a second to show a window, if the progress bar it embeds has completed before the second has elapsed, so there has to be a delay there, and so forth. The task is so quick it can really not be presented to the user without the presentation being slower than the actual task.
I think it comes down to good UI+UX, which in this particular case becomes bad UI+UX.
You don't necessarily know in advance whether it'll take 1 second, but I think a strategy that would work is: after 1 second, if the task is not done, open a progress bar, and if the task completes immediately after that, display the completed progress bar for 500 milliseconds or something before closing it.
If it was being shown and hidden again as fast as the task starts and finishes, it would be just be a flicker on the screen, which would confuse the user.
Maybe the really inexperienced users, but it makes sense that something which happens quickly, will mean a window that also appears and disappears quickly. This is why Windows has options to disable animations and other effects, and everything does feel noticeably more responsive when they are disabled --- windows and menus appear and disappear instantly.
I think an 'in-line' style of alert can work well in this situation, like a status bar but closeable. It can be dealt with at a time most convenient to the use, still provides feedback. It could even just display the output of a CLI command with a given flag, even for a small subset of 'blessed' commands if necessary.
I think it comes down to good UI+UX, which in this particular case becomes bad UI+UX.