That isn't true. Writing speed comes into play whenever anything is saved. My second 3DS (owner didn't format before pawning off) also came with a digital Kid Icarus Uprising (comparing with the one I have on my main 3DSXL) and a generic no brand Class 10 SDHC card. The game saves on every menu press (Sakurai
) and during idol toss. I see first hand that the copy played off the generic card takes a lot longer to move between menus and the idol toss screen hangs for longer.
But Class is primarily about reading speed and those are writing speed related issues.