It can be really difficult trying to quantify what a “slow website”  really means. Its easy to spot a slow site but arriving at some metric  that helps you monitor attempts at improving performance or better  still, a tool which actually makes some sensible suggestions…

Firebug and Page Speed
Using Firebug and Page Speed to test page loading

Well, now there is one by using a combination of Firefox addons:

FireBug - a general web developer type of tool

PageSpeed - an addon addon that not only measures page loading performance but  gives you several suggestions on what could be improved and also what is  working well

I am told that Google are now using performance metrics in their page  scoring so these tools can even help your SEO efforts. If you are also  trying to monitor the effects of a caching tool such as Squid or Varnish  or even Apache in front of your website then also add livehttpheaders to your list of Firefox addons

livehttpheaders - gives detailed info on objects downloaded and can help monitor cache hits