What`s faster?

What is faster using one multivertex polygon of split it to more GL_TRIANGLES.

I would say it depends on the OpenGL-implementation. Some triangulize polygons, so it doesn’t make a difference, some actually draw >4-vertex-polys, then it might be a little faster.
The primitive of choice for performance though is definitely a triangle strip. You can split any polygon into one or more triangle strips, and from my experience, they’re the fastest OpenGL primitive.