For any website, use a profiler like the ones from Jetbrains.com or red-gate.com. They tell you which methods are taking a long time. You will probably find out they are database calls. So use SQL Server profiler and the at the time when the site is slow, inspect the sql statements which went to the database. If it's a stored proc, you will have to inspect and performance test each sql statements to determine the slow one. Then you will have to find out why it's slow and from there it's another world. Try using SQL Server tuning advisor if you don't know how to optimize sql queries.

In Visual Studio 2010 you will be able to attach the profiler to the web server process while it's running and inspect it.

Check out the profiler and tracer from eqatec.com. They are free.