A friend, who is much more knowledgeable than I when it comes to hardware, said that the Lightning cable isn't capable of sending huge amounts of video data across the wire. Is this true? I'm not sure, but my experience using Astropad on the Pro backs it up. It was pretty laggy in two ways:
1. the stroke lags behind the brush by a second. This is alleviated by the purple line that immediately forms.
2. the stroke is drawn in as a series of low-quality pixels. a second later, it gets re-drawn hi-res.
The combination of the two delays was super distracting to me, and made it hard to use the Pro for drawing for more than a few minutes.
Maybe if they switch to USB-C in future models it'll be faster? Or maybe it's software? I'm not too sure.