Humans do not perceive the passage of time in a linear way. This, coupled with the irregular behavior of progress bars, causes human perception of process duration to vary. An understanding of which behaviors perceptually shorten or lengthen process duration can be used to engineer a progress bar that appears faster, even though the actual duration remains unchanged. This paper describes an experiment that sought to identify patterns in user perception of progress bar behavior. The results are then analyzed to classify behaviors that perceptually speed up or slow down process execution.