Quote - But we still can completely disable the raytraced preview, can we ?

If you don't open it, it will not do anything.

Quote - But seriously, basically the only things useful for me among the new PP-2014 features will be the Firefly speed improvements, but if they come at the cost of slowing my OpenGL preview down, even those would be useless.

The ray traced preview is independent of the OpenGL preview. It's in a separate window and runs in a separate thread.