Because the microSD card slot is using a USB 2.0 bus, it is limited to a max speed of 60 MB/s, but it is half duplex, so you are probably only going to get 30-40 MB/s speeds. That means that you are wasting your money if you buy anything faster than a 50 MB/s UHS-I card.
See: What are the specs of the Librem 5 SD card reader?
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