Here are some articles that discuss some potential solutions to rendering speed problems.
Even though the following article discusses Flamingo Performance Tuning, the techniques can be helpful for any renderer you use.
Flamingo Performance Tips
The following article discusses troubleshooting. Often there is some bad geometry that's causing problems while rendering.
Bad Objects
Mesh settings seem to be the single most important thing you can change to help with rendering speeds.
Mesh FAQ
It appears that VRay is becoming very popular for rendering large architectural models. For more information on the VRay plug-in visit:
V-Ray for Rhino - Rhinoceros - Related Products and Services
www.asgvis.com - Home
Another thing that can be a problem is the amount of detail and features in the model that have to be meshed. If there are some things that won't be seen in the rendering, turn them off. This includes small details that will be so small that they won't be seen anyway. If these details are visible in the model they have to be meshed. This is what takes a lot of resource.