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?
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):
What is the performance difference in those jQuery Lookups?
When we drill down into the PurePath on both recorded sessions it is easy to spot where the difference comes from:
I’ve said this many, many times on this blog: Element lookups by class name or any lookups not done by ID or TagName are EVIL in Internet Explorer. IE8 at least provides an implementation of querySelectorAll which the latest versions of frameworks like jQuery support – but be aware of some restrictions and concerns.
My new friend: the Compatibility View
Compatibility View is definitely a huge help for developers and testers. It allows us to test the same page on the same machine with two major versions of IE. For testers this is nice as they can run the same set of tests against the same installed browser but in different modes (however – I still recommend using two real, different browsers on different machines – because – in the end – the compatibility view is not the real thing). Use tools like the Dynatrace AJAX Edition to analyze the difference performance behavior of your pages.