What do the interfaces and speed classes mean?
UHS (Ultra High Speed) is the current interface standard for SD and microSD cards, with specific ratings like UHS-I, UHS-II and UHS-III denoting maximum transfer speeds. UHS-I cards have a max speed of 104MB/s, while UHS-II and UHS-III can reach 312MB/s and 624MB/s respectively.
On the Steam Deck, Valve opted for the UHS-I interface. UHS-II and UHS-III cards are backwards compatible with UHS-I, so you could use them in the Deck, but you’d be limited to UHS-I speeds; as such, theres’s little point in spending more to get the newer interfaces. Besides, game launching and loading times depend on non-sequential read speeds, which normally don’t come close to the advertised maximum sequential speeds anyway.
Another spec you might see printed on microSD cards is the speed class, like U1 or U3. Rather than general maximum speeds, these represent minimum sequential write speeds: U1 is 10MB/s, U3 is 30MB/s. Again, these ratings don’t have any real bearing on how fast your Steam Deck games will load, though it might be worth plumping for a U3 card if you’re using your Deck as a full-on desktop replacement, where faster write speeds will come in handy more often.