So this may be a noob question, but I've got a video set up with nodes, and I go to render, but blender makes 2 passes, one is the scene (which had been previously used for smoke simulation but doesn't have anything in it now) and then the compositor. It slows it down a lot, and I can't find anything online about how to only render one part of the blender project, wether it be the 3D scene, the node compositor, or the VSE. Help please?
Delete the renderlayer node from the node setup, and then blender won't render the 3D scene.
If there is anything in the VSE or the compositor, by default blender will prioritize rendering it over rendering the scene unless you explicitly disable it in the render settings:
If use nodes is enabled, then blender will only render the 3D if it's needed for the composite. (i.e. there is a renderlayer node using the scene as input)
I usually attach a Viewer Node into the node setup. Then in the UV/Image Editor, open the Viewer Node and then save the file using F3.