Re: "as fast as the fastest PCs."
It will probably work fine for most users, but I run rather heavy (and well parallellised) image processing code, like stacking 44 1,000-frame 6Mpixel monochrome uncompressed videos (250+ GB of data) to create 44 6 Mpixel panes to stitch into a 100+ Mpixel lunar mosaic. I would be very curious to see how much time that takes on the iPad Pro. On my laptop it takes some 12 hours, on a Core i7 desktop it is quite a bit faster.
Regarding the MacBook Air: no SD-card slot is a definite deal breaker for me. If I want to transfer a lot of photos from my camera to the laptop, popping the SD card into the laptop is the easiest and fastest way by far