In no way did I suggest you would need a full frame to do this testing. If you want to know if a certain type of rendering is slow you can test your various types of rendering, off screen, at initialization time and find out which ones are slow.
Unless you call gl.finish() which blocks you cannot test the time it took to render something. You do not want to call gl.finish() because it blocks. You may test performance characteristics up front, which tell you jack squat how long things will actually take to render troughout the use of the application. Timing how long something took to render, is the most accurate way to figure out how long it took to render.