[userbinator]

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?

[gilgoomesh]

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).