> You have multiple reflections and refractions. That mean that you have tons of
> rays to compute. Add the aa and the small bumps, you need lot of subsampling =
> lot of time.

yes, but looking at the original scene for the first time, I'd never have guessed it!

but you're right, there are rays passing through 3 cups with IOR and normals, as well
as reflecting off the other cups in the scene.  It's insanity squared!

> Try adding adc_bailout 0.01 in the global_settings.

I'll try that.  As well as some no_image, no_shadow trickery...

> It will cause a small decrease in quality that can go unnoticed, but will
> accelerate things as the ray tracing will often stop much earlier.

And I didn't even go with media for blurry shadows, area_lights, radiosity and caustics!  
I wonder how much time it'd take in an unbiased renderer such as Indigo or Luxrender...