I have culled the responses from the thread to make it easier for those wanting to learn about optimizing scenes. The comments are related to the scene in that thread and will vary from project-to-project. HTH
1/ Ray Depth, Reflection Depth, and Shadow Depth settings are important. What are the settings? Ray Depth and Shadow Depth set at 6 as recommended by Per-Anders here: http://forums.cgsociety.org/showthread.php?f=95&t=404342, post 8. Reflection Depth is default 5.
1/ too many omnis, light flattens scene - better to use one ambient light.
2/ area lights for sconces - if needed, use carefully controlled spotlights; a square spot with a good falloff for a good clean and quick result.
3/ bevel edges to give geometry thickness.
4/ control reflections and refraction ray depths (Render Settings, GI, Diffuse Depth) - if scene only needs a few bounces, try a setting of 4.
5/ blurry reflections/refractions - results in slowdowns.
6/ area shadows - results in slowdowns.
7/ overlapping shadows/lights in general - results in slowdowns.
8/ AA - adjust the tolerance just to the point where it gets rid of any visible aliasing, but no lower than that as lower settings will increase render times.
9/ GI settings - try reducing further or even just switching it off altogether if your use an ambient light - result pretty similar to GI.
1/ Ray Depth is very important.
2/ many lights with subtle or low intensity settings (%) washes things out and kills performance.
2a/ explore using vew larger area lights.
2b/ consider that omni lights cast 6 shadow maps whereas a spotlight only has 1 shadow map - the omni shadows are a big memory and speed killer.
3/ Pick either GI or ambient illumination. Using both together doesn't make sense and hurts speed. In this case with many omnis, adding tonnes of flat light and having that flat light then bounce adding more flat light.
4/ optimize the models using the same material and that are in final location - connect them into a single object. It will reduce the object count and make the scene use less memory.
1/ keep the GI but lower the settings to eliminate artifacts and use a spot of AO to help enhance the effect especially at wall/ceiling intersection if lighting is blowing out the scene.
2/ area light/shadows will drastically slow things down as will reflections and blurriness and displacement (tone down AA samples and shadow samples).
3/ lighting this scene, keep the fixed lumens and use a single omni ambient togther with GI AO.
4/ make use of individual object comping tags for sample settings, AA settings and GI settings.
5/ global AA settings for speed - BEST and SINC and leave the other settings as the defaults. To speed up rendering, push the Threshold setting up to maybe 15-20% or higher. Try turning the min/max down to 1x1/2x2. Keep the global setting on default but use individual render tags to change exact properties.
6/ AO? I use it in 99% of my rendered archi works.
1/ too many lights. - start with the minimum number of lights and work from there.
2/ more important is attention to model scale makes the image real.
3/ check the following:
3a/ color of chrome and the reflection color.
3b/ glass refraction as you can adjust this to get a clear or opaque edge.
3c/ fresnel used in the glass mat?
3d/ with GI, remember to turn off the auto light (Render Settings>Options>Auto Light).
1/ take care with the ray depth number.
2/ with GI use the Illumination channel of the material editor - make selection tags and affect them different materials with different values in the Illumination channel. This will lower the GI calculations and render times.
1/ reduce the number of lights to make a difference.
2/ refine the illumination model of thematerials - blinn, oren-nayer according to their "diffuse" behavior.
3/ pump up the values for the gi receive field, according to the placement of the objects in the scene:
floor objects reflect back 30% of the lighting
wall ob jcets 50%
ceiling objects 70-80%.
4/ render at 1/6 or 1/10 of final size, save the gi solution, and then render the final image with the save solution - it will be faster than the "prepass size" option.
If there are any mistakes, let me know and corrections will be made. Thanks to all that offered responses.