The only obstruction in the scene might be the construct if it is still visible.  The scene could be using up memory.  However, we don't know if material shaders have been simplified on figures and objects in the distance to reduce the file size overhead.  Too many lights can impact render time, as well as transparency maps for the hair and tree.  The best advice is to render the scene in layers, background, midground, and foreground.  This involves making all objects and figures invisible except the background and render the image once.  Next, make the background invisible and make the middle scene objects/figures visible.  Don't change any settings and render.  Finally, make the middle objects/figures invisible and the foreground objects/figures visible.  Again, render.  After each rendering, you want to save the scene with a transparent background, except the background image as PNG.   Open each image in an image editor and compose your scene with the 3 images.

Large scenes need to be handled differently than a straight still image render.  Background figures and props need to be low-resolution models to converse on the memory overhead.  And textures may need to be modified as well to reduce the file size.