The final cause for the unaccounted for time is simply because there could be 3 seconds of time between the Oracle client process and the user's experience, which is the missing component of the true end-to-end user time.

In a client/server architecture, there should be no time between the Oracle client process and the user's experience. However, with modern web architectures, this is not the case. Referring to Figure 5-13 again, the time between the Oracle client process and the user's web browser would be represented by unaccounted for time. This is your clue that the true performance issues reside outside Oracle's realm.

If the Oracle database is deemed guilty until proven innocent, I encourage you to either acquire or develop your own end-to-end response time monitoring system. This is not something you can throw together in a few hours. Indeed, there are companies based on providing infrastructure solutions6 to understand the user's experience. But it can be done using creative scripting, information from Oracle's performance views, and data from the networking group, and placing bots on a few strategically located PCs.