It would be cool if someone would do some benchmarking of the Librem 5’s microSD, using hdparm and dd, as described in this article and this one. It might give us some idea whether the USB bus is the limiting factor. For example, if the read speeds are 15 MB/s on a card that supports 50 MB/s, then the USB bus is probably not the limiting factor, since it should max out around 30-40 MB/s.