What is limiting the LTO Tape write speed? How can I observe the ideal listed speeds?

I am testing out a new Quantum Ultrium 8 Tape and Drive. I see the listed maximum uncompressed and compressed data rate, respectively, are 360 MB/s and 900 MB/s. How can I see this level of performance most simply? I was trying this:

dd if=/dev/urandom of=/dev/shm/random-data bs=64M count=16

end=$((SECONDS+60))
while [[ $SECONDS -lt $end ]]; do
    dd if=/dev/shm/random-data of=/dev/nst0 bs=1M count=1024
done

I got an average speed of 126 MB/s. When I tried to test the compressed data rate, by filling up another file in /dev/shm/ with 1GB from /dev/zero and doing the same loop, I got an average of 282 MB/s.

Is there another simple way to see the expected data rates (or at least just the uncompressed speed)? What is causing the limitation I’m observing?