Slow video performance with Ubuntu 12.04 32-bit

I am currently facing an issue where the video performance of QtWebEngine on Ubuntu 12.04 is very slow - it seems to happen with both Flash and HTML5 video.

The videos lag and drop frames to the point of being just about unwatchable. It seems to happen more clearly with HD video than non.

As a test I played the same videos with the Chrome browser and they looked good. I figure this rules out hardware.

In fact, while I was originally testing on hardware, I've since moved to doing it in a VM and see the same effect: QtWebEngine is playing laggy but Chrome plays fine.

One reason I point out 12.04 is because I have run the same QtWebEngine-based browser (this is really a simple browser, just using the WebEngineView QML element and displaying pages; and as a sanity check, I ran the simplebrowser example and see the same effect) in 14.04 and the performance is much better.

As a further test I upgraded xserver-xorg-core (and all of its dependencies) on the Ubuntu 12.04 machine, to the version from Trusty 14.04, and performance again got much better. This seems like a strong hint. This seems to suggest that the problem is in the software stack and not with hardware.

But since I'm not comfortable running a newer xorg-server in this case, for various reasons, I'm looking for other solutions.

Is this a known issue with 12.04 and 32-bit? Is there any particular commit around which might address this?