Making storage expandable complicates all of that.
It really doesn't. You set where you want a certain type of file (music, photo, video, etc) to go. And every time thereafter that's where the file resides.
And the only way a card MIGHT slow performance is if your running some massive app off the card...which people generally shouldn't do.
Never, ever had a phone slow due to SD storage...even going back to the old SIIx which wasn't a powerhouse to begin with.