https://wccftech.com/steam-deck-ssd-downgrade-cost-functionality-gameplay-unaffected/

As we reported yesterday, while Valve has been increasing Steam Deck production, they’ve also quietly downgraded a key component going into the machine. Some (but not all) 256 GB and 512 GB Steam Deck models will now ship with a PCI express 3.0 x2 SSD instead of a PCI express 3.0 x4 SSD. As you would expect, an x2 SSD offers around half the writing and reading speed of an x4 SSD. The Deck website says this change will have no effect on “gaming performance,” but people were understandably skeptical. Surely, this change would come at some sort of cost?

The answer is, yes, the SSD change will indeed come at a cost, but according to comments Valve provided to PC Gamer, it won’t be a big one. Basically, file transfer speeds will be slower in rare cases, but due to various hardware bottlenecks, the Deck wasn’t really using the full bandwidth of the 4x SSD most of the time and thus gameplay should be largely unaffected.