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.
"Use a progress indicator for any action that takes longer than about 1.0 second."
https://www.nngroup.com/articles/progress-indicators/
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.