Astropad is a very cool app from former Apple engineers designed to let you draw into Mac apps by connecting your iPad or iPhone to the computer via USB or Wi-Fi. You can then draw into most Mac apps directly on your iOS device's screen, using your finger as a replacement for a professional graphics tablet. In fact, it's too good that people are replacing their Wacom tablets with Astropad and an iPad.
I've been using Astropad quite extensively and it works like magic, really.
There's very little discernible lag, courtesy of the proprietary Liquid engine providing nearly three times more fluid responsiveness than Apple’s AirPlay. That's why I'm happy to report that in its most recent version 2.0 update, Astropad is now even faster and more fluid than before and a lot less taxing on the battery.