Help me understand these render times.

Those Things

I put this scene together, and did a test render before I added the buildings for the final version. Time to complete render was about 1 hour and 30 minutes. Not too shabby for 5000 pixels across. So I went ahead and added the buildings. Exact same render settings, same resolution, changed absolutely nothing. Time to complete render about 35 minutes. I'm quite used to longer render times being the result of adding more geometry and textures to the scene, so this was unexpected. Anyone know why this would be? I have the new volumetric ground fog (with the exact same settings) activated in both versions, but only the second version has the matte fog feature also activated. Does matte fog decrease render times? 

FSMCDesigns

Hmm, maybe because the light had nothing to really bounce off of in the first image and they did in the second, thus acheiving the goals faster

Spacious

Maybe your GPU released a bunch of vRAM between the two renders?

Those Things

FSMCDesigns said:

Hmm, maybe because the light had nothing to really bounce off of in the first image and they did in the second, thus acheiving the goals faster

I wondered if that might be the case too.  

Those Things

Spacious said:

Maybe your GPU released a bunch of vRAM between the two renders?

I suppose this is also possible!