How Much Faster is Fast Enough?

This paper reports on two experiments designed to further our understanding of users' perception of latency in touch- based systems. The first experiment extends previous efforts to measure latency perception by reporting on a unified study in which direct and indirect form-factors are compared for both tapping and dragging tasks. Our results show significant effects from both form-factor and task, and inform system designers as to what input latencies they should aim to achieve in a variety of system types. A follow-up experiment investigates peoples' ability to perceive small improvements to latency in direct and indirect form-factors for tapping and dragging tasks. Our results provide guidance to system designers of the relative value of making improvements in latency that reduce but do not fully eliminate lag from their systems.