A very good article written by Mr. Jason Himmelstein on the key things to note before starting the SharePoint performance troubleshooting.
Click Here, for the detailed article.
Read this white paper written by Eric Shupps, a renowned SharePoint MVP that specializes in SharePoint performance to learn how SharePoint deployments are a combination of applications, services and databases where delivering a scalable, dynamic and efficient portal implementation can be a challenging task for even the most experienced IT professionals. You’ll also learn how you can make improvements that can be made within or outside of the SharePoint infrastructure.
Another Article by Mr. Jason Himmelstein SharePoint Troubleshooting: A Little Data Goes a Long Way.