My understanding is, to speed things up on exterior glass, you should uncheck ‘affect shadows’. This is, unless you want your glass to cast interior shadows. Is this correct?
affect shadows is a fake caustic. It allows direct light to pass through the glass, which otherwise is impossible unless you use caustics. I doubt it affects the rendering performance that much.
I remember Vlado saying it slows exteriors down a lot.
Bobby Parker www.bobby-parker.com
Because VRay has a harder time to cleanup (and so takes longer), on the interior parts of there’s no light going through the windows
I really doubt that if you did two renders, one with glass (and affect shadows on) and one without glass it would be any different, with exception of the reflection / refraction time of the glass.
“Another slowdown point for this scene besides the trees is that “Affect shadows” is not turned on for the glass materials. This causes the interior of the building to be lit entirely with caustics from the environment map which needs a lot of GI samples to clean up. Best regards, Vlado”
Just the opposite of what I was thinking
Yup, exactly. I had to do a double take when you said affect shadows would be slower. Confused me for a second 
it comes back to what I said If affect shadow is off, then brute force will try to trace real caustics, which will be very very slow. You can either use caustics photon map, or just simply use affect shadow 
In what circumstance would you disable it? Would interiors look better with real caustics?
when you use real caustics exactly.
Even worse! If you don’t have GI caustics turned on, a glass material with affect shadows turned off casts a solid shadow from your glass!
both are on by default, correct? I’m not near my PC to check.
Affect shadows is now on, caustics aren’t.
Would be great if there was a better more consistent understanding of caustics. In all my years using V-Ray I’ve never been able to feel like I can “control” them or understand the best settings to use (Maybe it’s because I’ve never known how patient I have to be ie setting the subdivisions to 100k etc)
I have asked for better caustics method for a long time. Perhaps some day it will happen. But I have a feeling that if it was easy, we’d already have it. I’m not sure how corona or other new renders deal with this issue, but I clearly remember in mental ray and arnold same sort of “affect shadow” function. Even in maxwell they have specific glass shader to do that since real caustics are just such a pain.
But I wish instead of having caustic photon map which to me is as old as a dinosaur and getting it to work on complex large scale scene is a real pain. So I really wish there was a more user friendly globally working caustic method that didn’t require caching of any map, etc etc. BF caustics suppose to do that, but they never look good or work as you’d expect. Just take too long to clean up.