Check your Raycast Parameters too.Like render region division, region sequence etc.I prefer ADAPTIVE DMC rather than Adaptive Subdivision….Too many lights can also cause overkill of the rendering.The rendering parameters (global switches) to make sure all polygons are modeled correctly for test rendering so that you know the types of shaders you placed individually and you can trace easily which materials could possibly cause the rendering longer. Before you place all the shaders, first try to override the materials in.Irradiance and Light Cache (save to file), skip the computation when using the same sets and scene over and over again.Post process could be short the rendering workflow, professional composition programs like Autodesk’s Combustion, Photoshop, After Effects, Nuke, Fusion… could help you a lot.Instead of using dof in physical cam, Photoshop can do the trick with zbuffer channel or plugins like Alien Skin’s DoF and DoF Generator PRO by Richard Rosenman.Be wary of glossy reflection and frosted materials.(case to case basis).It's up to you if you want to drink and eat and sleep until the rendering finish. Be wary of iteration levels when making curves/mesh smoothing.I always shutoff my antivirus while rendering (or don’t install one at all).Get rid of the undo window, especially commands on the list that eat up memory….Know the difference between copy and instance… Simple but effective.Use high polygon objects only when they are close to your camera.Try to use your own materials rather than prepared. A simple diffusion, bump and reflection(with. Material wise: avoid using too much glossiness and highlights.

If you render your final image at 3500px you don't need more than 4000px image anywhere.

Remove unwanted objects from your scene.

Don’t put too many subdivisions on your shadows (as much as possible).

If you are using Vray, always proxy the ones with high polycount.

The more polycount, the slower the rendering.

Limit your polycount to minimum as much as possible.

Rhino 5 render settings