I am having a similar problem
the strangest thing is that the rendering takes way longer than playing back the entire project (?!?)
changing the buffers doesn't change too much.

I've discovered sometimes adding track automation for volume or pan or whatever, places nodes that are a long way out (like 30 minutes or something ridiculous), and even that's all silent with no Midi events it lengthens the render time severely.
 
If that's what you see, just right click that last node and select "delete node" and you'll find the render time go back to something reasonable.