Long patient dwell time (i.e., the time between patients arriving and leaving the clinic) has been a long-standing issue in the eye clinic at The Hospital for Sick Children. By applying the Lean principles of eliminating waste and enhancing flow, we achieved a 26% reduction in the mean patient dwell time over an eight-month period. Importantly, the average time a patient spent with healthcare providers (value-added time) increased from 21% to 31%. In this paper, we summarized our experience by illustrating how an implicit mental model (conscious or unconscious conceptual framework from which we understand the world) pervades in the healthcare system based on deeply held but unexamined assumptions that arise from heuristics (general rules of thumb) and biases; how these assumptions can be tested by objective data; and how we can build a new mental model based on objective findings to improve the healthcare system.