It's the internal Internet Explorer browser that is rendering the web page. I don't know why some pages would be slow, and I'm not aware of a setting that controls or limits how long it takes, but you might experiment with IE's browser emulation and other feature control options (see IEBrowserEmulation at
http://www.kinook.com/Forum/showthread.php?t=5097 and the MSDN link there).
You can also hold down the Alt key when selecting an item to prevent it from using the internal viewer to display it, or remove .html and .htm from
Tools | Options | Browser | File extension to display in internal browser view and open externally (Ctrl+J).