If you are requestion pages from machines other than the tests are run on, the user/sys times are not relevant (if the requests are done the same way). The user/sys time is the amount of CPU time the tests take to run. If a process is waiting for something in a syscall (i.e. waiting for network input) the scheduler usually switches to anytother process, so the actual CPU time for that process will not increase. The wallclock will increase, since that is the actual amount of physical time that is spend (the clock on the wall).

In other words, if you want to measure the responsiveness of your servers, your current setup is good, and you can more or less disregard the CPU time on your test machine and just watch the wallclock time. You should make sure you get no interference from other processes on your test machine and servers, though.

One of the advantages of measuring in CPU time is that if your machine is really busy running other processes, the CPU time for a test will not be influenced by it. But it doesn't really work "cross-process" and "cross-network".