Painting with polygons in animation

It’s been bugging me for past few days. Is there a method using layers and node system to render objects painted with polygons apart from rest of the scene without rendering everything over and over?
Lets say we have a quite huge scene with many high poly objects. We would like to make an animation and place low poly forest in the background. My idea was to make this forest using painting with polygons. It worked quite well but the thing is to make the effect satisfactory I had to use 11x or 16x sampling. Forest was beautiful but the problem was rendertime. I had to render the whole scene 11-16 times extending the time one frame needs to render to 20-30min. Needless to say considering an animation that’s out of the question. So I tried to do something with layers and nodes to render only the forest with motion blur and the rest of the scene only once but since I’m new to blender (using it for only past 2 months) I don’t know how to do it or is it even possible.
Maybe someone knows the answer or better a solution?