Test the write speed of the SD card by writing 1 GB of data in 10 blocks of 100 MB:

dd if=/dev/zero of=./TestingFile bs=100M count=10 oflag=direct

Test the read speed of the SD card by using dd to read the entire TestingFile file generated from the last command, in 10 blocks of 100 MB each:

dd if=./TestingFile of=/dev/zero bs=100M count=10 oflag=dsync

Source article: https://linuxconfig.org/how-to-test-sd-card-speed-on-raspberry-pi