The SD Association has this page explaining the speeds, and from what I can see, it’s just to guarantee speeds with certain bus types. Most devices currently use a UHS-I bus, and the older speed classes cap out at pretty pathetic numbers, so they’re kind of useless. For the steam deck, any of the higher-end cards are going to be about the same, and the IOPS are going to be more important in terms of loading times anyway.
The SD Association has this page explaining the speeds, and from what I can see, it’s just to guarantee speeds with certain bus types. Most devices currently use a UHS-I bus, and the older speed classes cap out at pretty pathetic numbers, so they’re kind of useless. For the steam deck, any of the higher-end cards are going to be about the same, and the IOPS are going to be more important in terms of loading times anyway.