The Devel module provides a suite of tools for the Drupal theme and performance developer. Two major components of the Devel module include helper utilities that monitor performance of your site, and tools for themers that allow for quick and more accurate theme development.
Developers can use the module to monitor all of the database queries that occur as each Drupal page loads. This tool will show you how many times the database query executes on each page when it loads. Knowing this is essential when monitoring performance because it allows you to find out which queries are repetitive and creating potential bottlenecks or other performance issues especially on a large site or multisite environment. As the module project page points out, you do not want to see queries running multiple times on the same page. The development component of the module also tells you how long the queries take to run. This can give you detailed information on which queries and functions may...