When you try to open a website in Internet Explorer 11, you may experience slow rendering when the website is displayed in Internet Explorer 5 Quirks mode.
This issue occurs particularly for websites that use complex table structures.
This issue occurs if CSS rules are dynamically applied to different layers or parts of table structures. Those changes may trigger a complete layout recalculation of all child elements down the DOM tree.
Quirks Mode has not received any performance improvements to maintain backward compatibility. Therefore, poor performance may be detected in this scenario.
This behavior is by design when Quirks Mode is used. We recommend that you use the most current Windows Internet Explorer standards mode, which is not affected by this issue.
Most web browsers will switch to Quirks Mode if a webpage uses nonstandard HTML document declaration. Apart from most non-Microsoft web browsers, Internet Explorer uses a Quirks Mode rendering engine that's not based on a Standards Mode rendering engine. The Quirks Mode rendering engine for Internet Explorer is based on Internet Explorer 5.5. It was first introduced with Internet Explorer 6 for backward compatibility with legacy webpages.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.