The first step is to establish whether the problem is client-side or server-side.

A good YSlow grade indicates the problem probably isn t clientside. YSlow checks to see that you don t have too many objects on the page, that you have minified your javascript/CSS etc. It does not evaluate the performance of your network or server.

Using YSlow/Firebug, check to see how long it takes to load the actual HTML of your page. If that is taking a long time, then the problem is almost certainly with your server, network or server-side code.

To rule out network issues, compare accessing your site from the server itself to accessing it over the internet. If it s a lot slower over the internet the problem could be network-related.

If it s not client-side or network-related, then it s either that your server is struggling for resources or that your code is slow (perhaps because the amount of data it is mananaging has grown).In that case, check the server logs and run a profiler on your code (on a development server but with a copy of production data).

Tools like YSlow will point out some opportunities for optimization but they don t acrually measure performance and they don t look at how long it takes for things to happen.

Try something like WebPagetest which will give you a browser-view of the page loading and you can work through the waterfall to see where the time is going.

If you are seeing timeouts then it s probably a back-end problem (will be pretty clear in the waterfall) and you re going to need to instrument your server to figure out where the time is going. If it s a dedicated server or VPS then you can install something like New Relic and it will point out the problem pretty quickly. If you are on shared hosting then you re going to have to add logging to your app directly (there are plugins that can do this if you are running something like Wordpress).

The first place to look would be the server logs , that should provide you a clue as to what is happening and how much time a request is taking in general .
If the server is returning fine and the page is taking long because of client side code , you might want to use the Firebug profile to profile your page and find out more . Hope this helps .

Want kind of pages are you trying to load? Plain html or scripts like PHP? If plain html I guess its your hosting company.