Originally Posted by AaronP
This is just a performance problem on new (relative to the GPUs 173.14.* supports) desktop environments, right? The legacy drivers get support for newer kernels and X servers (though I'll admit I'm behind on adding support for xserver 1.11), but not things like performance improvements or support for new features.
I'll see if I can give it a try on one of my legacy GPUs, when I get a chance, to see if there's anything obviously wrong. It's likely that mutter/gnome-shell is just hitting a rendering path that was not very well optimized back in the 173.* days, so don't get your hopes up.
No this is just the shape problem that got fixed in the non legacy drivers, but the legacy drivers apparently never got this fix. Gnome Shell adjusts the output and input shape during animations (which cause X to call validateTree) which the nvidia driver did not handle right back then (hit some kind of weird performance problem).