How to render certain edges I been looking for this everywhere online and still found no answer. I'm trying to find a way to use freestyle but it slows my render down. How can I use freestyle for certain edges while not rendering the rest of the mesh at all. Like seams almost I just don't even want the render to even see the rest of the mesh. All I want is the outline of certain edges to render quickly. I also have most settings in render tab set low.

Right now I have a cube with outline rendering at 0.15  to  0.20 

My question is if there's a way to mark edges to get rendered while the rest of the mesh doesn't even get seen, I think that will speed up the render exponentially.
You may use the Edge Mark option of the Freestyle to define what edges would be rendered.


Enable Freestyle renderer in Render header-->Freestyle panel. Set the line thickness as you like.


Select the edges you want the Freestyle to render, then press Ctrl+E-->Mark Freestyle Edge to make them renderable.


In Render Layers header-->Freestyle Line Set panel check the Edge Mark checkbox. Uncheck the Silhouette, Border and Crease checkboxes to make the contour of the object renderless. 


Here's the render result:


You can go further and render only selected edges. With the vertices/edges selected in Edit Mode duplicate them with Shift+D and make them a separate object (P-->Selection). Move the original object to another layer and hit render. 


Here's the result:


Not sure if it is what you're looking for. Let me know if I helped you.

UPDATE: If you want to render only the edges give the object a Transparent shader in Cycles. The Freestyle settings should stay the same as in the examples above.