I use Air Display quite a bit with a LiveReload web browser or an additional terminal in the second window, and it works great. Lowering the resolution speeds up the response, but I've never experienced a 4 second lag.
According to Avatron, Apple does not allow them to use USB. [1]
>> Can Air Display connect using a USB cable...
>> No. In order for iOS apps like Air Display to access the USB connector, its developer must be part of Apple’s Made for iPod/iPhone/iPad (MFi) program. As Avatron is not a hardware shop, we are not eligible for this program and therefore cannot support USB.
However, Jailbreaking the iPad is a workaround since you can create a USB private network for Air Display to connect. [2]
According to Avatron, Apple does not allow them to use USB. [1]
>> Can Air Display connect using a USB cable...
>> No. In order for iOS apps like Air Display to access the USB connector, its developer must be part of Apple’s Made for iPod/iPhone/iPad (MFi) program. As Avatron is not a hardware shop, we are not eligible for this program and therefore cannot support USB.
However, Jailbreaking the iPad is a workaround since you can create a USB private network for Air Display to connect. [2]
1: http://www.avatron.com/support/faq-air-display/#toggle-id-14
2: http://hijinksinc.com/2010/06/01/use-air-display-over-a-usb-...