Kixum, thank you! I've got it and am about to set it rendering. Let's see what I get.

In the mean while I rendered a fake bounce scene (So my spot for the window shadows, plus a huge amount of low intensity lights all over the place to fake bounce - about 75 of them. I didn't alter their color yet, so the whole thing is still gray, but I plan to change that eventually to have a more believable color bounce.) with AO only for indirect light. My file (not very small this time, but not full resolution either yet) rendered in 33 minutes. So big improvement over 4 hours, but the quality is clearly not the same :(...

 

P.S. this should also give you an idea of how populated I want my final scene to be: each of these chairs should have a fully clothed person sitting in it, plus there would be one more fully clothed person by the board.