ok, I took a look, here is a couple of notes:
1. Don't put CoronaLights into the windows. They are blocking the environment/sun which is then sampled for nothing. To get the lighting from environment it is enough to leave the windows free, or put a geometry there with CoronaPortalMtl for additional speed boost (but the same visual result).
2. The weird multiple rendering was due to multipass effect turned on on the camera
3. Do NOT use gamma 1.0, it is simply wrong. If you like the look of gamma 1.0, then render it with gamma 2.2 and darken it in postproduction. Corona assumes in some places max is set up for linear workflow (rendering without it is really just a thing of the past, it does not make sense to support it in a modern renderer like Corona). Not using LWF could result in some unexpected bugs. Although it looks like the scene works fine.
3a. BTW: Corona overrides 3dsmax gamma settings for output by default, so you are using 2.2 anyways ;)). But with the risk of some bugs.
4. It is possible the reflections from the black material stopped converging at all because Alpha4 is using an obsolete random number generator that had this problem. This will be fixed in the next release, you can also try to change it in A4. I dont remember the exact names of other RNGs in A4, but something like "5D + PRNG" should work better.
5. The most important: the slowness of the black material is not actually due to the material, but due to the slowness of the entire scene. Your renders have extremely low number of passes for the time elapsed. There are usually 2 reasons for that:
a) low rays/s number - this is usually due to some really crazy texmaps/geometry. This is not your case, milions rays/s are fine
b) high rays/sample (you need to press stats button in VFB to see it). This is usually because of too bright materials/too complicated geometry with opacity, and this is indeed your case. Most materials in your scene have albedo (sum of all its colors) near 1. It is generally not possible to render scenes with albedo = 1 in an unbiased way. Scenes with albedo close to 1 can be rendered, but with extreme render times, such as yours.
tl;dr: you have too bright materials in an enclosed space, that are slowing down the render. You should make your materials darker and compensate by increasing exposure.
Perceptually white material can be made both by setting diffuse color to 1 and using light with intensity 1, or by setting diffuse color to 0.5 and light to 2. The latter is much faster and usually also more physically correct. CG artists have a long tradition of using too high albedos. See this chart:
http://en.wikipedia.org/wiki/File:Albedo-e_hg.svg - most real materials reflect less than 30% energy!
But, if you do not wish to change the scene, then just simply decrease maximum ray depth in the main renderer settings. Value about 7-10 will decrease the time to 1/3.
