I don't understand. If you're testing how fast pipes are, then I'd expect you to measure throughput or latency. Why would you measure how fast something unrelated to pipes is? If you want to measure this other thing on the other hand, why would you bother with pipes, which add noise to the measurement?
UPDATE: If you mean that you want to test how fast pipes are when there is other load in the system, then I'd suggest just running a lot of stuff in the background. But I wouldn't put the process dedicated for doing something else into the pipeline you're measuring. As a matter of fact, the numbers I gave were taken with plenty of heavy processes running in the background, such as Firefox, Thunderbird, a VM with another instance of Firefox, OpenVPN, etc. etc. :)
Oh, wait. You mean that this "cat </dev/urandom >/dev/null" was meant to be running in the background and not be the pipeline which is tested? Ok, my bad for not getting the point.
UPDATE: If you mean that you want to test how fast pipes are when there is other load in the system, then I'd suggest just running a lot of stuff in the background. But I wouldn't put the process dedicated for doing something else into the pipeline you're measuring. As a matter of fact, the numbers I gave were taken with plenty of heavy processes running in the background, such as Firefox, Thunderbird, a VM with another instance of Firefox, OpenVPN, etc. etc. :)