IE9 on Window 7 slow (excessive rendering)

This is a very bizarre any very frustrating issue so please bear with me for the full explanation.

PREEMPTIVE NOTE: This is not a question of how to improve DOM/CSS for performance. You will see when you read the frustrations.

We have recently re-worked out website to be cross-browser (e.g. < !DOCTYPE html >)

After all the pain HTML and CSS re-factoring, we thought things were fine. However our website which has a pretty heavy DOM (including jqgrid 4.5.2) is incredibly sluggish to the point of being unusable. Example: clicking to select a row takes 10 seconds until it is highlighted and its check-box is checked.

Frustrations:

1) This sluggishness is not universal. Only IE9 suffers from this. IE8, IE10, FireFox, Chrome are all speedy!

2) IE9 itself if changed in debugger to IE8 documents mode becomes speedy! But not in its own IE9 mode.

3) While we are noticing the some sluggishness, the SUPER sluggishness (to the point of unusable) is only on specific machines.

4) Of course, one of those machines is a client.

The Question:

What could possibly unique about specific machines to cause super sluggishness? What could cause these machines to suffer more than others?

Things we've checked:

1) Windows 7 fully updated.

2) We do not have Windows update KB2976627 installed

3) We do not have Windows update KB2670838 installed

4) Disabled all Add-ons

5) Stress again that we disabled Shockwave Flash Object.

6) Tried disabling "Software Rendering" in Internet Options so that the GPU is used.

7) Reduced Windows display quality settings to run for maximum performance.

Additional Evidence:

When the display area gets smaller, responsiveness increases. This happens when the browser window itself is made smaller or when dragged partly outside the screen boundaries. Also when performance improves as DOM elements removed/hidden.

DynaTrace shows a huge amount of time spent on rendering.

Webpage content:

There is ONLY: HTML, CSS2, JavaScript, JQuery, simple images.

There is NONE of the following: Flash, video, CSS3, loaded third-party web content.

Machines tried:

OS: Windows 7 Enterprise N 64bit RAM: 2 GB CPU: 2.53 Ghz (2 processors)

OS: Windows 7 Enterprise 64bit RAM: 8 GB CPU: 2.67 Ghz (2 processors)

IE9 Version:

9.0.8112.16421 64-bit IE9 Update Version: 9.0.31 (KB2977629)

We tried uninstalling update KB2977629 with no improvement.

Repeat of Question:

What could possibly unique about specific machines to cause super sluggishness? What could cause these machines to suffer more than others?
Aaron MirskyJava Team LeadAsked:
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Big MontySenior Web Developer / CEO of ExchangeTree.org Commented:
I've recently run into this same scenario on an old classic asp site that was built 10+ years ago who wasn't all that familiar with html standards :) After adding some DOM intensive functionality, it slowed down to a crawl in IE9. Because the html was malformed, it through the page into Quirks Mode which was causing this issue. If I manually put it back into IE9 Standards mode, it worked (but messed up some html which I had to fix).

To test if this is your problem as wel, open up IE9 and hit F12 (opens the dev tools) and see what mode it's opening up in. Alternatively, you could put the meta tag below as the first meta tag in your head section:

<meta http-equiv="x-ua-compatible" content="IE=9">

This'll force the browser into IE9 Standards mode
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GaryCommented:
What kind of video card is on the machines - are the drivers up to date?
Is there a link you can provide?
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Aaron MirskyJava Team LeadAuthor Commented:
Thanks for the responses.

Sorry, cannot provide a link. This not an online service, but a web app used on internal company servers.

We are aware of the quirks mode. We have a requirement for clients to disable compatibility view in their browsers. Sounds weird I know, but given the type of clients we have its a demand we are able to make. So in dev tools I do see that the page is in "IE9 Documents mode". Like I said I sounds like a IE9 engine specific issue because if in dev tools i switch to IE8 documents mode the page runs fast again.

BTW, always forcing IE8 documents mode with the meta tag is not an acceptable option.

I am checking with the client regarding the video card and drivers.
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Aaron MirskyJava Team LeadAuthor Commented:
I discovered the explanation. As I feared when I phrased the original question JQGrid was a distraction (red herring). The real cause was answered in a Stack Overflow article.

In short, our page contains a lot of rounded corners (i.e. border-radius) styling. And there is a bug in IE9 that causes the rendering engine to choke on this. Once this styling was removed, everything was speedy again!

This also explains why IE8 browsers and even using IE9 with IE8 documents mode were fast. IE8 does not support border-radius so it didn't fall into this issue.
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Aaron MirskyJava Team LeadAuthor Commented:
This completely resolved the issue. If absolutely require rounded corners and must support IE9, then taking other approaches recommended. For us, we can live without them.
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