For the past few days, I've been testing an iOS tool called Duet Display. Eric Cheng of DJI clued me in on the $15 app, and it's one of the more interesting and useful iPad utilities I've used so far. Simply, it allows you to use any iPad--whether it's an old 30-pin or current Lightning cable model--as a second screen for your Mac or PC. Yep, it's platform agnostic, and the desktop client is free. Using a 9.7-inch or 7.9-inch tablet as your secondary monitor may not sound like a great idea, and it's not something I would use on a regular basis. But since I keep both a laptop and my iPad in my backpack for most places I go, this is something that may have a lot of utility for frequent work travel.
The ability to use an iPad as a second display isn't new--iOS apps like Air Display have granted that ability for years. But those apps rely on a tethered or shared Wi-Fi connection, which limits the quality and responsiveness of the extended display image. The host computer is essentially sending compressed video over to the iPad, and that requires a lot of bandwidth. Duet Display uses a wired connection, so the only limiting factor is the host computer's ability to render and compress a desktop to send over the cable (Duet Display is admittedly a bit of a CPU and power hog, if you're running on laptop power). I was impressed by how good the desktop on my iPad Mini looked, and how responsive the cursor was as I moved windows between screens. It's not exactly zero lag, but darn close.