One of the most important aspects of performance tuning and even programming in general is knowing when to stop. Getting your web site or product into the hands of the customer is after all the goal of all this. If you sit around and fiddle with it for months, nobody is using it.

Once you have located all the low-hanging fruit, further optimization tends to get exponentially harder for less and less gain. The time you spend on it compared to simply buying another server or two is usually not worth it. Stop and move onto the next project.

Or better yet, go home, spend some time away from these beastly computers and spend some time with your wife and/or kids