In the accessibility settings on Yosemite I have enabled the "Reduce Transparency" setting multiple times. The reason I am turning it off is to try to fix the issues causes with the menu redrawing when multiple monitors are connect and eventually slowing down the machine to a crawl. Each time I turn it on and go back in and check it has turned itself off.
It seems to turn itself off if the external monitor is connected.
Why does this setting not work?