More comprehensive testing:
uSD card: SanDisk Ultra C10 U1 512 GB
read test with: hdparm -t /dev/sda
write test with: dd if=/dev/zero of=test bs=1048576 count=200 conv=fdatasync
(speeds in MB/s meaning decimal MB, not that it makes a huge difference)
With the card in a USB 3.0 card reader connected to a USB 3.0 port in a middle-of-the-range x86 computer …
read: about 85 MB/s
write: about 37 MB/s
With the card moved into the Librem 5 …
read: about 7 MB/s
write: about 10 MB/s
read speed rises to 11 - 12 MB/s if tested with
dd bs=1M count=200 iflag=direct if=test of=/dev/null
where test is a 200 MiB file on the uSD card. Create with:
dd if=/dev/zero of=test bs=1M count=200 conv=fdatasync