How slow is FrameBuffer.getOutputBuffer()?
When I used this method, I did not see any major performance changes.
Depends on the renderer, the graphics hardware, the OS, the drivers...when using the software renderer, it's free. When using the hardware renderer, it isn't. But there's no fixed delay that it causes...it simply depends on a lot of things. If you can avoid it, then avoid it.
Ok.
How mutch faster is the ogl renderer than the software renderer?
That depends on the scene and if compiled objects are being used or not. I would say something between 10 times and a 1000+ times faster.