Certain performance issues can be tracked down to a specific aspect of the server configuration. For example, it is common practice to use the assign-name NameTrans directive in obj.conf to apply rules based on request URL pattern. More and more assign-name directives get added as time goes by and specific requirements come up, and it is not unusual to see configurations with the number of assign-name directives running into hundreds.
Each assign-name directive introduces a regular expression comparison, which can be CPU intensive. Hence, as the number of such directives increase, performace start getting affected.
For more information on this specific topic, see Chapter 6, Scalability Studies.