I don't really have idea about that so I'd like you to give me some advice if you can.
Generally when I use Selenium I try to search the element that I'm interested in, but now I was thinking to develop some kind of performance test so check how much time take a specific webpage (html, script, etc...) to load.
Do you have some idea how to know the load time of html, script etc without search for a specific element of the page?
PS I use IE or Firefox
You could check the underlying javascript framework for active connections. When there are no active connections you could then assume the page is finished loading.
That, however, requires that you either know what framework the page uses, or that you must systematically check for different frameworks and then check for connections.