Performance-based architectural design pursues building performance objectives like energy efficiency to guide design decisions. Nevertheless, these informed decisions are usually made according to performance simulation software results, which are quite effort-consuming and tedious. Meanwhile, this low feedback efficiency blocks architects' mind flows from creating new design alternatives. Therefore, the current authors develop a digital platform by coupling Internet of Things (IoT) and Mixed Reality (MR). It streams and visualizes real-time sensory data of indoor illuminance into an MR glass HoloLens. It enables students to intuitively observe spatio-temporal illuminance fluctuation when manually modifying the physical shading device model. Therefore, they can quickly learn how design decisions influence indoor illuminances. At last, in the validation experiment with first and second year students in an architecture school, we compare this platform with two other teaching methods and platforms. Experimental results identify that the IoT-MR platform can effectively attract students and increase their learning effects.