Here is a final render, which takes 2h54m on my 6-core Phenom 1100T:
This rendertime is absurd of course. Two days ago the same render was taking mere minutes, perhaps 20.
Diagnosing the issues led me to tweak/hide all lights and reconstruct the sun/sky and portals as well, with no change, so I removed all but the sun/sky directional (FG off) light to figure out what was up. This one does not hang up, and renders in 54 seconds:
Turning on FG, renders in 1m22s:
Turning on the portals and interior vanity lights... Stopped at 13m42s, as rendering still hadn't began (per the Output Window's verbosity):
Isolating the scene further, it turns out that the glass light fixtures are the cause of the problem, with or without Photon GI enabled. With the glass fixtures off but the point lights still on, the scene renders in 6 minutes.
So next test, replacing the light_surface mia_mat_x with a standard "thick glass" preset yielded the same hangup prior to rendertime. It wasn't the more complex shader shown in the first render here, but a basic one. I modeled the lights myself and there appear to be no geometry issues, and normals are all facing properly as well...
Has anyone ran into such a problem? Any ideas would help at this point! A nearly-3 hour render isn't normal for my work (done scores of these types of scenes) and I'm rather stuck on this scene at the moment.