[OWaz]

With Mavericks I was okay connecting to two 30 inch Dell displays. Once Yosemite came out, I would face problems with resolution on the displays switching or even better I would be forcibly logged out. So I gave up on dual monitors for a while until recently but now I face CPU usage issues.

[tillinghast]

This. I was only pushing out to two 24" displays and I routinely experienced the forced logout. A touch disruptive. So now I sit at a desk with one dead monitor. Heck of a job, Apple.

[Tehnix]

Disabling transparency will most likely help tremendously on the CPU load. In System Preferences -> Accessibility -> Display -> Uncheck "Reduce transparency".