Virtual and Augmented Reality enables unprecedented possibilities for displaying virtual content, sensing physical surroundings, and tracking human behaviors with high fidelity. However, we still haven’t created “superhumans” who can outperform what we are in physical reality, nor a “perfect” XR system that delivers infinite battery life or realistic sensation. In this talk, I will discuss some of our recent research on leveraging eye/muscular sensing and learning to model our perception, reaction, and sensation in virtual environments. Based on the knowledge, we create just-in-time visual content that jointly optimizes human (such as reaction speed to events) and system performance (such as reduced display power consumption) in XR.