A client of ours recently contacted me with the question: We use Keynote, WebPagetest and dynaTrace AJAX – but we get different results with these tools/services. WebPagetest tells us that our page is very slow – but dynaTrace on my local machine does not. What can be the problem here? What’s the difference?

I took a look at their page on my own laptop – running WinXP and IE8. I saw a small JavaScript performance hotspot but nothing major. Then it occurred to me that WebPagetest is probably using a different browser which made me notice a feature of IE that I haven’t yet explored enough.

IE Compatibility View

Internet Explorer 8 introduced a feature called Compatibility View. The intention of that feature was to allow web sites that were optimized for older browser versions to display correctly and it also allows developers to test what their site looks like on an older version of IE. From a performance analysis perspective this feature also allows you to verify if your site has been optimized for multiple versions of IE – without needing a second machine that runs an older browser.

Comparing Page Speed

I ran the page that was supposed to be slow twice – once without and once with the Compatibility View. The following two illustrations show the dynaTrace AJAX Timeline View of the same page (first one is normal IE8 – second is the one using Compatibility View):

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