I haven’t tested this enough to know if it’s a real hidden feature, or just me placebo-effecting myself, but I swear it takes way less time to wake up my TV (Samsung) if I press the little TV button on the ATV remote 5x really fast.

Like if I press it once, the ATV wakes up and usually the TV does, but often showing the wrong source. 5x fast consistently wakes the TV to the right input.