While we typically associate latency in applications with network performance, I’m guessing Gruber is referring to responsiveness to user input. Is this a huge concern for app developers? Not that I’m particularly aware of.
Responsiveness in a touch-based platform is undeniably and unequivocally important. It’s probably one of the most important things, since the user input is so direct. in a non-touch scenario, user input is abstracted through a pointing device, be it a mouse or trackpad. When the user is touching the onscreen controls, it feels more like a direct command, and requires the content to perform and display naturally.