I seem to remember watching a real time video transcoding demo, or something like that, on an iPad Pro and the dev demoing it saying it was the I/O that made it possible. Desktop machines with much faster processors would choke due to bandwidth limitations, but Apple can custom tune every aspect of the iPad architecture and aren't limited to standard, generic interfaces between components.

The original 5k iMacs were a similar case in some ways. It was only possible in an all-in-one design, because there wasn't a standard video connection at the time that was capable of the throughput.