What's your story?
Your chosen art direction is a soft, pastel dream rather than a photorealistic rendering, so there's not much to say about technique. The shapes and relationships of the structures are well-elaborated, but photoreal aspects are missing - largely in part due to the lack of model detail necessary to convince the eye. However, to properly detail this project would be a stupendous task. This looks like a modeling exercise rather than an urban design. At least i hope it is.
As for rendering time, making reference to the pixel output size is of minor significance relative to the rendering speed of the application. It's all about polygons in the model and the rendering quality features selected. You can always tell this because when you ask for a rendering, the system works for quite a while without producing any image. This is because it is resolving the raytracing "solution." Then it renders the solution. The more polygons in the model, the longer this solution and its subsequent rendering. This model appears to have a very few polygons relative to a real project, except in the trees.
In improving rendering the task is to study photos similar in aspect to the rendering planned. Seeing a photo would help you fix problems like the over-prominent parking slot markers and other over-detailed elements.
My observations are:
Impending disaster: the airliner is on a collision course with the balloon. Airliners at low altitude far from an airport are sinister. We fear that the pilots are chanting. Besides, everybody knows that terrorists cannot resist destroying spheres painted with orange and blue stripes. Like a red cape to a Spanish bull. Many illustrators place entourage elements without considering the implied story. Also, considering the fog, the airborne elements are distractions. RULE: Be logical or leave it out.
Landscape: The larger trees might be the right height, but their parts are way out of scale. Since the foliage has the only roughness in the scene it makes the trees too prominent. Your best foliage is on the left in the mid-ground. With a model having schematic detail, perhaps you should use translucent spheres on sticks to represent trees. Uniformity counts.
Light: Very incongruous shadows. Can't explain why. Airliner has oversharp shadow, foreground elements shadow too diffuse. I suggest using the sun sun shader rather than realistic sun. This will produce a more uniform shadow that will help in expressing the different levels of the scheme that you complain about. Light and shadow inform.
Color: The colors and geomentry of the bright green, ochre and orange fields adjacent to the project are distracting. Add more fog or suppress the field colors. While i have previously observed that "THERE IS NO SUCH THING AS WHITE!!™" so often that it is a well-known slogan, your work inspires me toward a new slogan that ridicules grey. We think that asphalt and concrete are grey, but there's a range of color in those materials that can please the eye. I'd make the road surfaces darker and rougher, or lighter.
Surfaces: Roughness adds gravity, slickness look plastic. Rough, GOOD. SMART. Shiny, BAD. STUPID. For instance, in North America we have trucks dangling mudflaps with chrome silhouettes of reclining, long-haired women. I think that the chrome women are supposed to be lures for real-life "shiny" women. Shiny gals look at the man in the pick-up truck, see the chrome silhouettes on the mud flaps and are attracted. They say "Look!! On the mud flaps!! Shiny women! Let's meet that guy, I think he's a doctor."
Entourage: Remove cars totally like the Japanese do for their architectural photography OR add more cars to explain that there IS some life to this place, even if the cars are blocky. Make an accident at one of the traffic circles. That would be photo-realistiic.
Dwight Atkinson