i've been on the 10.10.4 beta and its been much better.

I'm on 10.10.4 and am solid with wifi, but plugging into two monitors with the hdmi and thunderbolt ports runs my cpu full load. It's become extremely annoying.

That begs the question, what is taking up all the CPU?

Is the GPU not doing what it should be, and somehow the OS has fallen back to software rendering everything?

Most drawing in OS X is done by the CPU. The GPU is used for compositing (layering the views drawn by the CPU over each other) and additional special cases (CoreImage effects, OpenGL, etc).

I have no information on what is going weird in 10.10.4 for OWaz but it's far more likely to be a Thunderbolt driver issue than anything else (i.e. simply related to pushing data over the cable).

has it ever been any other way?

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.

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.

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

That beta releases have bugs ?

Yes. I suspect that has always been the case.

Where can the 10.10.4 beta be fetched from?

Thank you!