Database maintenance required
It is a far too common occurrence as a SQL Server consultant to have a complaint of poor performance and when you investigate you find that there is no regular database maintenance occurring. Databases need regular maintenance. You need to rebuild or reorganize your indexes, update your index statistics and recompile your stored procedures - regularly. This is could be nightly or weekly, depending on your maintenance windows. Although I am not a fan of the Database Maintenance Planning Wizard, it is an easy way to accomplish setting up this maintenance. I find stored procedures and SQL Server Agent jobs more flexible and there are a number of these available for download if you want to go down that path. The best that I have found and now use as a standard is Ola Hallengren's suite of stored procedures and jobs. They are excellent, as I have blogged about previously.
But whatever you do, do maintenance!