The pv tool allows you to view the throughput of the data flowing through it. After installing the package, you could run a command like the following to see the write rate:

$ cat /dev/zero | pv > /media/some_usb/tmp123
203.1MiB 0:00:02 [100.2MiB/s] [ ]

For reading, perform the opposite operation using the newly-created tmp123 file:
$ cat /media/some_usb/tmp123 | pv > dev/null