Q: Zoom (Accessibility feature) is extremely slow with multiple monitors [OSX 10.10]

Same issue here...Apple please fix this ASAP. This is unacceptable!

Maybe not a Thundebolt issue but definitely an issue related to the computer when having an external monitor connected.

Did you guys notice that the sluggishness also applies to Mission Control when switching desktops?

This is so annoying! It makes me feel I'm using a 386 PC!!!

Yes, there is a very serious issue with overall performance when an external display is connected. Over time the system becomes extremely sluggish. You can also turn on dark mode and see the menu bar icons flash when you switch desktops, also attributed to the external display (see: Menu bar icons flash when changing Desktops).

Same problem on my 2011 iMac (note that I only have the internal iMac monitor). Accessibility Zoom and Mission Control were smooth like butter up until Yosemite. Now both are frustratingly very choppy and sluggish.

Ok - I installed X11 for some other reason and restarted the system. Zooming is now much better but not that good the it used to be with Mavericks. I am not sure if the X11 installation has something to do with it - I guess not. Can somebody just try a clean restart?

yep right after system restart (without doing anything else) it is indeed much better (but not as good as before Yosemite)

I think I can already feel it's slowing down as I open all my usual apps, will update this post in couple hours

can confirm I also have this issue on mid 2013 27" iMac. I am blind in my right eye and left eye has a bit of difficult seeing so I use the zoom feature A LOT!. hope this gets fixed.

actually it wasn't that bad yesterday evening, still worked OK. Let me try it again tonight, so that'll be roughly after one day system uptime.

I have a similar issue.  I am visually impaired and noticed in Yosemite when I am zoomed in, in fullscreen, and moving around a lot, it freezes.  I had posted a forum earlier about this but no responses.https://discussions.apple.com/thread/6621430

I have the same issue with two different macs, even without plugging in an external monitor.

iMac 27" 2010

Macbook Pro Retina 2013

I am seeing this issue as well on Yosemite.  It has been there all the way from Beta 1 to final release.

MB Air 13.3" 2013 model.

Zoom lags extremely when connected to Thunderbold but is also horrible just on the 13.3".

If zoom is attempted when some sort of video is running, sometimes all graphics freeze for a minute.

Hi,

This issue occur since you switch from the internal graphic card to the dedicated graphic card, even with no external display.

When you you plug an external monitor you automatically switch to the dedicated graphic card whatever the connection of the monitor.

Have to re-install mavericks, no comment...

That totally makes since as to why this is an issue. So in essence the theory is that the dedicated graphics are the culprit? When running on internal graphics (i.e. Intel) it's not an issue but running on dedicated graphics (NVIDIA, AMD, etc) causes this. Time for some kext hacking...

I'm having this same issue, and not only with an external display either. Every time I try to zoom, it's extremely laggy and sends my fan rpm way up.

I'm using a 13" mid 2009 MacBook Pro, and the zoom feature has worked fine on all previous versions of OS X.