File size bears no relation to the 3D generation time since a small file can still generate complex form [and a big 3D file] through executing GDL code.
Hardware weaknesses of your system notwithstanding, you can try several things to speed up 3D views:
• download the polygon counter to establish the true 3D file size… polycounter accessory here
• isolate and hide elements that might generate complex 3D like trees and roof tiles.
• arbitrarily divide the project in half using the marquee. Image each half individually to establish if there is a difference in rendering time. You might have a corrupt element that makes phantom calculations.
• reduce the finesse of the rendering - eliminate cast shadows and vector hatching - anything that adds to image complexity.