Performance is not a problem until it is. Then it is a very big problem.
The root cause of performance problems can be varied and multiple. Sometimes the causes are obvious, more often not. Oftentimes what initially seems to be the primary problem is just a red herring.
Some of the causes of poor performance include:
Finding & fixing the causes of poor performance takes experience in the full technology scope of the application.
Fortunately, performance problems can often be dramatically reduced or eliminated.