Duet is the first high performance solution to use your iPad or Android device as a second display for your Mac or PC – unlike other second display apps, Duet Display has zero lag.
accomplish this by turning your mobile device into a second display using the USB to 30 Pin, lightning, or USB-C connection. It is far more stable than Wi-Fi, especially for people on the go. You don’t have to worry about your 802.11n network, your router settings, whether you’re on a VPN, or if you’re downloading or uploading too much data to use your display.
Source: Duet