Spinner vs progress bar is not short vs long operation, it's non-progressive vs progressive operation (though you may opt for a spinner if a progressive operation is consistently, predictably very short).
In the software world, designers need to be engineers (or work very closely with engineers, very early in the process). We recently had designers give us a nice looking progressive indicator for a non-progressive operation that happened to take a somewhat predictable amount of time. Business loved the look and visual feedback. Then we got use cases where the operation could be instantaneous. They didn't want to skip the progress animation because they liked how it made the app feel like it was doing something, and also if the user got the result instantly, it was "jarring". So now, even if the operation finishes instantly, we show the animation, wasting the user's time.
In the software world, designers need to be engineers (or work very closely with engineers, very early in the process). We recently had designers give us a nice looking progressive indicator for a non-progressive operation that happened to take a somewhat predictable amount of time. Business loved the look and visual feedback. Then we got use cases where the operation could be instantaneous. They didn't want to skip the progress animation because they liked how it made the app feel like it was doing something, and also if the user got the result instantly, it was "jarring". So now, even if the operation finishes instantly, we show the animation, wasting the user's time.