If the site is running slowly or timing out due to SQL timeouts being reported on the site, it may be an indication that an SQL database reindex is needed. Here are the steps necessary to perform this task.
If the hardware is that where the server is dedicated to SQL, large space, powerful CPUs with greater than 4 GB RAM, chances are maintenance has not been done to the database itself. It's easy to perform these maintenance tasks below.
For more database maintenance information, see this article.