Actually, what you're calculating is load-time-from-last-hit + wait-time-before-next-hit + latency-for-next-hit. What I have seen done is to have a hook into the Apache preprocessing create an entry in some sessioning database, using some session ID. Then, a hook in the Apache postprocessing will write the time for that session. The difference between those two times is the processing time. In fact, I'm a little surprised there isn't a hook in mod_session for this ...