That's possible of course (well, 'of course' outside the Apple world) but not very convenient in my experience except if you use the same SD card all the time. Everything else doesn't fit well with a portable device.

The question remains, however, how well the Surface manages data an SD cards, i.e., can you save everything you like on an SD card including apps?

Perf will not be great to the card though. Depends what you install or store there, or if it even can be installed there. SD cards are fine for general media storage, but anything perf intensive you don't want there.

Presumably the SD port will cope with Class 10 or UHS-I and so transfer at HD video rates. There can't be much call for higher rate transfers for most users; it's not like you're going to use it for data gathering from a hadron collider.