Drawing a subsection of ofCamera

I am trying to draw only a subsection of an ofCamera’s view. Normally, I would do this by drawing the camera into an ofFbo, and then calling fbo.getTextureReference().drawSubsection(), but my window is so large (7680x720) that using an Fbo is incredibly slow. Is there anything that would allow me to control the camera’s view? ofCamera::begin(ofRect viewport) looks promising, only I can’t find much documentation on how that works, and playing around with it has proved rather unhelpful.

Looking further into ofCamera, it seems like there is only matrix and related math going on, and the camera itself doesn’t actually have a texture. I was able to use a viewport to do the equivalent to fbo.getTextureReference().drawSubsection().