For folks who really care about site optimization and performance, this is a big deal. Especially if you're hired to help a client optimize the page-load time of their web-site, but services like Pingdom lie and report junk.
Here's an example; imagine this is your homepage:
<html>
<head>
<title>My Homepage</title>
<style type="text/css">
#header {
background: black url(/header.png) no-repeat top left;
}
#homepage {
background: white url(/homepage.png) no-repeat top left;
}
#aboutus {
background: white url(/about-us.png) no-repeat top left;
}
#contact {
background: white url(/contact.png) no-repeat top left;
}
#footer {
background: black url(/footer.png) no-repeat top left;
}
</style>
</head>
<body>
<div id="header">header</div>
<div id="homepage">content</div>
<div id="footer">footer</div>
</body>
</html>
Ok, so you've got a header and a footer that will appear on every page. In addition, you've got a few other CSS selectors for the homepage, for your "about us" page, and for your "contact us" page. In a real web-browser, when you visit this page the browser will load the following images as referenced in your CSS:
- header.png
- homepage.png
- footer.png
- header.png
- homepage.png
- about-us.png
- contact.png
- footer.png
To further validate my findings, I used the Rackspace Cloud and allocated a new machine with a running web-server. I loaded this hypothetical homepage HTML onto it, and asked Pingdom to tell me how fast it loads:
Wrong! I'm not using aboutus.png or contactus.png anywhere on the homepage, so why should Pingdom factor the load time of those images into my test results? In contrast, using Firefox and HttpFox, I verified that real web-browsers only load the resources they need to properly render the page:
Here you can see the correct resources are loaded in Firefox: header.png, homepage.png, and footer.png.
Bottom line, you really shouldn't use Pingdom and other external performance analysis tools as "bibles" for improving your page-load time. Clearly they're only tools to help you identify obvious problems, and are definitely not a complete solution. For more accurate results, I would suggest using tools like YSlow and Google's Page Speed.
Cheers.
