how to know for sure what is the reason of web app poor performance

i work in an organization and we have a major web application which can have really poor performance sometimes .....some of it's features are really slow ....the company that has developed the app say there is something wrong with the network or poor performance is because of too many request clients sends to server....i have installed fiddler to check how long it takes for a request to reach the server but i am looking for something more solid,something more reliable to know the real cause of the problem ... any suggestion ? any helpful app out there?the application is developed in silver light