Quality of Service
In my experience, the main causes of SharePoint performance and response issues noticed by users are not a result of SharePoint being at fault at all. They are a result of network or SQL Server issues. In my early years, I would often be frustrated when trying to troubleshoot the vague message from the help desk that “SharePoint was slow.” All of the SharePoint and Windows event logs were fine. Eventually, some network or SQL Server guy would casually mention that they were having some performance issues that day. That SharePoint is so dependent on these two other tiers is now something I am well aware of, so it’s one of the first places I go if I think the quality of service problem is being caused by something outside of the application itself.