<p>Thermochromic smart windows enable passive solar modulation by adjusting solar transmittance with temperature variations of the thermochromic window induced by solar and environmental heating, offering a promising strategy to reduce building energy consumption and improve indoor environmental quality. While previous studies have focused extensively on optimizing transition temperature, hysteresis width, and optical contrast, the role of transition speed has long been overlooked. Here, a kinetic model grounded in unsteady-state diffusion is developed for small-molecule adsorption/desorption-based thermochromic systems governed by the Van’t Hoff relation, explicitly incorporating transition speed, transition temperature, and thermal hysteresis. A standard transition time is proposed as a performance criterion, enabling a quantitative correlation between transition kinetics and environmental fluctuation frequencies. Simulations reveal that transition speed influences energy savings, visual comfort, and thermal comfort in a nonlinear stage-effect manner. For such Van’t Hoff-type systems, when the standard transition time is below 5 hours, thermochromic windows can track hourly solar fluctuations, achieving optimal performance. At moderate speeds (200–5000 hours), seasonal temperature variations can still be followed, with energy, visual, and thermal performances reaching 64.0%, 80.0%, and 58.0% of those of fast-response windows. Beyond this range, slower transitions fail to match environmental dynamics, resulting in ineffective solar modulation. This work provides insights into the practical performance of thermochromic smart windows.</p>