@amosbatto, here is another proposal (please check it), slightly changed, for microSD I/O performance testing on Librem 5. Only important thing before executing commands is that those need to be inside related (present) directory/partition (and for some kind of consistency my SDcard was formatted to ext4), therefore as first:
cd /media/USER/microSD_path
Today I’ve used count=2048 (perhaps 4096 would be ideal), yet quicker count of 512 will do as well. Note that for test_data files you’ll need su rights to remove those (sudo rm test_XX_data).
Just for coarse comparison results with the same HP mx310 UHS-I U1 100MB/s 64GB microSDXC card (only W and R commands changed to test_PP_data) inside PinePhone were:
Write: 215.399 s, 10.0 MB/s
Read: 180.912 s, 11.9 MB/s
cd / … to test_PP_eMMC_data:
Write: 129.938 s, 16.5 MB/s
Read: 27.9539 s, 76.8 MB/s