What does ROOT use for 3D graphics? Is it only OpenGL or it uses another library also?

OpenGl is one of the libraries used. We also use X3D (see the View menu in a ROOT Pad). Root also has its own 3D graphics to render scenes in the Pad.

Were there any productivity tests (speed) of root with OpenGl, with x3d and with its own 3D graphics? (and if they were then where I can find the results?)

I personnally do not have numbers. For OpenGl it depends if you are using a native one with 3D graphics accelerator or not. Results can be very different. X3D is very fast in wire frame mode. But it is hard to compare with Opengl because it is based on X window and cannot benefit from hardware accelerators.