I hope u trying to find out the responce time of page load of web application..right.
if so use YSLOW add on to mozilla browser and find out page load time.
There are many things you could test (performance of a web application)
response time of page
performance of the gui
performance of the database/dataware house behind the application
functionalities stored in the application (calculations and stuff)
does the gui have a performant desig (p.e. how long does it take to search for an article, store it in your shopping basket and pay via credit card)
.... and so on
It really would help if you could tell us what kind of applikation you wonna test & how the test environment looks like.