What causes the site to render slowly in Chrome (when using IE9 user agent)?

Using the latest version of Chrome and the extension "User-Agent Switcher for Chrome", you can set Chrome to use IE9 as its user agent string. When doing so, theverge.com loads at least three times slower than the default UA or iPad UA. What within the theverge.com source code is causing this delay? Is this what is causing native IE9 to have poor performance with certain sites compared to native Chrome?

Test it out yourself. Get the extension from the Chrome store (http://spoofer-extension.appspot.com or https://chrome.google.com/webstore/detail/user-agent-switcher-for-c/djflhoibgkdhkhhcedjiklpkjnoahfmg) and load theverge.com. Then change the agent to IE9 and notice the significant delay. Most other agent load perfectly fine, but not IE9. Is something intentionally breaking it?