http://www.learnasp.com/freebook/learn/debug2.aspx
shows typical use of trace. No web.config involved.

when you run the sample @
www.learnasp.com/.../sqlclientdropdown.aspx
look closely the the "Trace Information" table.

The last row "End Render" left column labelled "From First(s)"
tells how long  the page took to execute.

Notice also that right column shows time between last trace point.

The beauty is unlike response.write you don't have to comment out after
finished debugging..
<%@ trace="false"%>
turns off all of this
=> without having to remove the trace statements from your code  <=

http://www.learnasp.com/freebook/learn/debug1.aspx
shows way to use in web.config that is totally optional.