Getting Astropad to run at peak performance can be complicated, as there are three potential performance bottlenecks: the Mac, the iPad or the connection between the two. In most cases, the bottleneck isn't the connection (USB or Wifi) but instead is usually the Mac or iPad.
Now if you have an iPad Pro and a very powerful Mac, the USB connection may be the bottleneck. In the case of the Retina MacBook (2015), which is the only Mac currently to have USB-C, it isn't powerful enough to take advantage of USB3 + Astropad.
The good news is we are working on major performance improvements to Astropad right now. We are further tuning Astropad for the latest CPU and the GPUs, so that you'll get an even more responsive experience. These software improvements will also allow us to take advantage of USB3 once it's rolled out to more new Macs that are powerful enough to drive Astropad at peak performance.
So in short, USB3 isn't a silver bullet. Instead, a combination of USB3, software improvements and ever faster hardware will improve Astropad responsiveness.