How's the GI in this game different from other games that use it, are they running more stuff on a deeper level in the lighting engine or something? Or is it a fully dynamic GI and not semi baked? There's a couple of games already using GI in different ways and those don't require nearly as much horsepower as QB.
The game's lighting engine is supposed to feature a fully dynamic and real-time Global Illumination. It was somehow necessary given the kind of really fast time-skipping (which also means change in lighting conditions) that can occur in certain situations.
Having only pre-baked GI, like in AC:Unity for example, for a whole day/night cycle for every environment in the game would lead to some crazy storage requirements, which in a way is manageable, but also massive data streaming on the fly when the lighting condition keep on changing. Thus the choice of a real-time GI.
The immediate consequence is the fact that it give us one of the most beautiful and above all accurate lighting we have seen in any game up until now. There is very few "cheating" here, apart from something like still using a screen space ambiant occlusion technique (I think) to replace real contact shadows and maybe a few pre-baked lights here and there.
But hey, it is not like this is going to change unless we move to a fully raytracing based render. Which is not exactly 4K/60fps ready at the moment if you know what I mean.
The other, rather obvious, consequence is the requirements for your rig to make such a cutting-edge feature running up to "PC standard". This somehow take us back to the Crysis time, back when a lot of new effects where introduced or improved, such as SSAO, dynamic environments (and breakable trees of course), loads of dynamic shadow etc. SSAO was a very taxing graphic option for quite a long time, and we will somehow have to wait a bit before our rigs manage to keep up easily with a real-time GI, nothing we can really do about it. Something like a "simple" double bounce Global Illumination IS going to murder our GPU, that is a fact.
(PS : I am not try to make excuses for the current state of the game regarding its performance, just try to give an explanation on why this game is supposed to be very taxing anyway)
EDIT : Snappy snoop, didn't manage to find their SIGGRAPH publication back then, thanks sertopico !