Why Does A New Mac Pro Only Support a Single Internal Drive? Every graphic-heavy application I have used has recommended storing files it uses on a separate disk from the boot disk (preferably an SSD), however with the new MP this isn't an option. The only way to utilise a second drive is as an external drive.

I'm assuming this isn't a problem in terms of performance, and from what I've read MPs are very performant, but why is this? Why can a Mac Pro get away with a single drive when most apps recommend using separate drives for boot and resources?
Thunderbolt 2 supports data rates of up to 20 Gb/sec. The newest SATA spec, V3.2, only does 16 Gb/sec. There should thus be no difference in performance between an internal SSD (connected via SATA) and an external SSD (connected via Thunderbolt).