Apple - Yosemite: Accessibility zoom + multiple monitors = poor performance

This is now fixed in OS X 10.10.2

There is a workaround posted on the Apple forums.

Pros:

• The mouse/key entry performance does "feel" significantly better, as does the momentum zoom.
• Zoom is per-screen, rather than both at once

Cons:

• It is still not nearly as performant as it was in Mavericks. The testufo.com site will still not render at 60fps while zooming.
• While this workaround does make multiple-screen performance somewhat better in Yosemite, it makes single-screen performance much worse! Single screen performance is already "perfect" using the default "Full Screen" zoom and this is a step backwards.

Instructions:

• In System Preferences-Accessibility, set the Zoom Style to Picture-in-picture.

• Click More Options...

• Click Adjust Size and Location
• You will see a rectangular magnified panel with an "OK" button in the middle. Drag the corners to the upper-left and bottom-right corners of the screen. Click OK.

Now, when you zoom, it is usable, but still noticeably slow.

Best workaround so far:

In System Preferences-Accessibility, select Increase contrast (which will implicitly select Reduce transparency). This seems to improve performance in some circumstances, but it does dramatically affects aesthetics. The slider on http://kraken.io is smooth while zoomed in. There are still some circumstances where zooming is a little sluggish, but it is not nearly as bad as before.

Edit: After several hours of running with Increase contrast enabled (and upgrading to Yosemite 10.10.1), I can say performance is now back to an acceptable level.

I'm still not thrilled about having to enable an accessibility feature I don't want or need to improve performance. I really don't like the appearance of Increase contrast: solid black outlines around text inputs & buttons, the zebra-stripe effect on lists, etc. is all a bit jarring coming from the subtlety of OS X.

I tried just enabling Reduce transparency (which gets implicitly selected when you enable Increase contrast) but that setting alone doesn't help: both need to be enabled to see any performance gain.

Screenshot: Normal appearance

Screenshot: Appearance with 'Increase contrast' enabled