Laptop users who occasionally need more screen real estate can benefit from an additional display. And there are quite a few iOS apps (with companion Mac apps) that let you use an iOS device as that display. But, up to this point, what they lack is speed due to the slow Wi-Fi signal that allows the two devices to communicate. A team of ex-Apple employees address this issue with Duet Display, an iOS app that works in conjunction with a Mac app and a wired connection between your Mac and iPad.
Just how bad is using a Wi-Fi connection? Try to play a web video on the iOS screen, for instance, and the experience is choppy at best. If you move that movie window around, the video will typically freeze and then lurch back into motion once you’re done dragging.