Interactive Health Information systems are often considered cognitively complex by their users, leading to high cognitive burden and increased workload. This paper explores if Think Aloud usability testing provides valuable input to effectively redesign a web-based Data Query Tool in Intensive Care and to reduce physicians' cognitive workload during system interaction. Pre and post redesign usability testing demonstrated a major reduction in the cognitive task workload after redesign of the tool. Classification of revealed usability problems by means of the User Action Framework pointed out that usability problems related to the cognitively planning of actions by system users foremost affected cognitive task workload. This result may support Health Information system (re)design efforts on how to tackle the system's cognitive complexity and in so doing improve on its usability.