Interactive systems can adapt to individual users to increase productivity, safety, or acceptance. Previous research focused on different factors, such as cognitive workload (CWL), to better understand and improve the human-computer or human-robot interaction (HRI). We present results of an HRI experiment that uses pupillometry to measure users' responses to robot movements. Our results demonstrate a significant change in pupil dilation, indicating higher CWL, as a result of increased movement speed of an articulated robot arm. This might permit improved interaction ergonomics by adapting the behavior of robots or other devices to individual users at run time. CCS CONCEPTS • Human-centered computing → Ubiquitous and mobile computing systems and tools.