For completely imperceptible computing, display refresh must be dealt within 50 ms, roughly. Input must be sampled, all relevant computations for the current rendered display must be computed, or in easily accessed storage, and all updates to the display propagated to the framebuffer. Sensitive humans can notice as little as roughly 50 ms of lag or jitter.
This means for a program dealing with highly interactive graphics that are linked to an input device, the core event loop must execute in less than 50 ms or so. Even with current blazing-fast machines, this is a tough challenge for anything complex. If this deadline is not met, potentially perceptible lag in rendering will occur. In a 3D rendered scene, the graphics may perceptibly hang or tear for a couple frames. This is perceptible by a human, though with sustained suspension of disbelief, we can mostly ignore it, much as we can ignore the various visual artifacts in 24 fps cinema.