So just to add some interesting perspective... Apple transitioning will be as seemless as PowerPC to Intel, it is just a simple recompile with xcode to generate a new Universal Binary 2. For apps that aren't ready, Rosetta is coming back as Rosetta 2 for real time binary translation, no good for real time video/audio but for some apps will probably be quite usable.
And ARM performance? Fujistu just toped the Super Computer 500 with an all ARM, no GPU super computer! It will probably get trumped next week with another super computer using the new NVIDIA GPUs, but even so that is quite an achievement...
Apple probably already have a prototype 64 core ARM SOC that they can build a dual or quad cpu desktop with, I only hope this isn't the begining of soldered ram in desktop (imac excluded) and a complete walled garden like ios.