I was kicking around the idea of using the iPad Pro for my mobile machine instead of my Surface Pro 3, but to be honest, I'm not sure about what tools are available and/or how well it would work on RAW files. I'm no pro, just a hobbyist but I do use Lightroom on my iMac and I typically load them on my mobile machine for a quick culling and editting, and then load them on my iMac.
You might want to take a look at Syncomatc and LrMobile. Even with USB3, my D810 files took a long time to load on the Pro. That's where Syncomatic comes in. If you shoot RAW+jpg with a dual slot camera, you're in business. On my D810 I shoot a small jpg to the sd slot. RAW to the CF slot.
Import the jpg files into Lr mobile and edit.
Here is where Syncomatic comes in.....
"After getting back to a real computer, import the raw files into Lightroom as normal, and Lr Mobile would automatically bring in the JPEGs which were processed on the road."
"Syncomatic 2.0’s new capture time option allows you to use the capture time to sync metadata and adjustments from JPEGs imported directly into Lr Mobile, and copy the work to the corresponding raw files."
Its important to note that crop adjustments will not sync. But for culling, flagging, rating and other basic editing it works.
Here is a link to the article that explains Syncomatic in depth.
Syncomatic and LrMobile ? Lightroom Solutions