If you were able to use your iPhone on a monitor with a keyboard and mouse, with a useable UI (not even really needed), it would be more than fast enough for anything you do.
Fast enough for anything you do, perhaps, not the case for everyone. Heavy photoshop/3d modeling/audio work/video editing/compiling (even light 'proper' gaming) on an iPhone grade processor? No thanks.
I really hope they don't switch to ARM anytime soon, the switch from PPC -> x86 was a PITA but at least it was a worthwhile switch, going x86 -> ARM seems pointless, especially in a very expensive 'pro' machine.