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.
For many users, the lack of speed many not be a big issue--if you're just looking for room to hold some windows and browse email, the slow refresh rate isn't going to be a big deal. But if you want (or need) a high-speed connection to your iOS display, Duet Display has the speed you desire.
Unlike its competitors, Duet Display works only over a wired (30-pin dock connector or Lightning) connection. When you first launch Duet Display, it installs a video driver, which requires a restart of the host Mac. (Uninstalling the app is a painless option, available directly from Duet Display's menu bar icon, that doesn't require another restart.) The combination of the custom video driver and wired connection means blazing fast speed for your iOS "monitor."
How fast? 30 or 60 frames per second fast. That troublesome web video? It plays smoothly, and keeps playing while you drag the window around. You can use touch to interact, if you wish--just drag via a finger or bring up Finder's contextual menu with a two finger tap.
Duet Display offers only two options (via its menu bar interface) for your iOS display: the frame rate (30fps and 60fps) and, if applicable, retina or "normal" display modes. In retina mode, the display is crisp and sharp. (There's no "use all pixels" mode as you'll find in similar apps; retina mode is strictly about the quality of the displayed image.)
Those two options also reveal my only real complaint about Duet Display as of today--it can be a real CPU hog. When set to display in retina mode at 60fps, the CPU usage on my 13-inch Retina MacBook Pro with attached retina iPad mini exceeds 120 percent. With this much CPU going to the extra display, I noticed some lag when switching between apps and launching new programs.
Setting the frame rate to 30fps and disabling retina mode drops that figure to around 30 percent, which is still quite high. If you're using Duet Display on a laptop, you'll probably want to be plugged into a power source. The developer is aware of the CPU consumption issue and is investigating ways to reduce CPU usage. (Any such improvements will be distributed in a free update.)
Some may view Duet Display's $15 price as excessive, but if you need a fast extra display on your Mac at times, $15 is much cheaper than purchasing an additional display for occasional use (not to mention how hard it would be to travel with one).
I was impressed with the speed and ease of use of Duet Display. If the developer can solve the CPU usage issues, it will become a compelling solution for road warriors in need of speedy extra display space. In its current form, though, I'd recommend it only for extended use if you're connected to a power source.