Poorly written queries can be and often are the main reason for slow server response time. They’re a common bottleneck in the process of delivering dynamic web pages. You probably can’t make every query super fast, but you can certainly improve many, starting with those that have the slowest performance.
There are generally multiple ways to query a database for the same set of results, but some will perform better than others. As queries are often the most time consuming part of the process of delivering dynamic web pages, it makes sense to know how to find which ones are most in need of improvement and, of course, what you can do to improve them.