This is why I've been arguing that Nintendo should use UFS cards, rather than MicroSD, for expandable storage. Nintendo could guarantee pretty high speeds for both game cards and internal storage if they want, potentially significantly higher than HDDs in sequential reads and orders of magnitude better latency/random reads, but that would only be of limited use if developers also have to account for off-brand $5 MicroSD cards which could be arbitrarily slow.
I can't find any info on minimum read speeds of UFS cards (Samsung claims their 256GB card can do over 500MB/s, but doesn't mention what the smaller capacities are capable of), however given that 32GB embedded UFS can achieve over 200MB/s reads, it's reasonable to believe that their card-based counterpart would at the very least be comfortably faster than HDDs. The cost of the actual UFS card slot would be relatively trivial for Nintendo, although the costs of the cards themselves for customers might not be, at least at first, but by adopting the new UFS card standard they could guarantee a far, far higher minimum for games to run off, which would potentially be very valuable from a development point of view.
