You need two software applications for these products to work: the first gets purchased, downloaded, and installed on your iPhone or iPad from the App Store; the second gets downloaded and installed on your Mac or PC. The desktop apps are free and can be installed on one or more computers. Once the pair of applications is installed and started, you connect your iPhone or iPad to the desktop computer using your cable and magic happens.
There is no lag between the desktop computer and the iPhone or iPad. Using the standard desktop display control software, you can change the location of the second display with respect to your desktop just like you would with a “real” second monitor, positioning it left, right, above, or below. There are a few other settings available, but all the default settings generally work fine. Touch is still available on the iPhone and iPad, so it is easy to drag screens around, select items, and so forth. Besides touch, it works just like any second monitor would.
I actually tried two different products: Duet Display and Air Display 3. Both products had the same problem: if I minimized iTunes, I would be returned to the login screen and all my applications would have to be relaunched. I contacted Duet Display and they quickly responded, indicating this is a known problem having to do with certain screen interfaces that iTunes uses. Although annoying, this problem is easily circumvented by not minimizing iTunes and just letting other windows open on top of it.
However, I did have a serious problem with Duet Display in that after running for a few hours, my computer would freeze up. On my iMac, I run a product from VMWare called Fusion that allows me to run Windows in a Mac window. The presence of Fusion is likely what caused this problem with Duet Display, but the issue for me was serious enough that I had to abandon Duet Display and move to Air Display 3. Other than the same iTunes issue, I had no other problems with Air Display 3.
You can learn more about these products at www.duetdisplay.com and www.avatron.com. Duet Display is $18.99 and Air Display 3 is $14.99. For me, Air Display 3 met my requirements!