I think you'll find that more often than not this problem can be solved through optimisation. My system isn't "upto spec" anymore really, and I hate buildings with lots of load doors because then I spend all my time waiting for the next area to load, then the next, and back again, and so on, so frustrating, but a place causing constant lag can be just as bad!
The place I'm hopefully going to release later today is a fairly big place with sleeping space for 17, and 4 coffins (not honestly sure is NPC's use coffins), and sufficient space for them to move around in. There are a number of areas in the mod, and aslong as you're in the Dark Brotherhood you can use them all, even if your not you should be able to use Davokiin's room without the guards feeling the need to behead you lol.
I've tried my best to optimise the place, and as a level 1 player (using COC from load screen to test the place) I get no problems with performance, when I use it with my level 91 character I get some of those annoying game stutters around wall torches and the few shadow casting lights, so I may swap out the torches for wall scones instead. I've split the place up with room boxes and then occlusion planes inside rooms so I'm confident that this large place wont make a large hole in any ones FPS, but that remains to be seen I guess when it's released, I am a beginner and mistakes are probable lol but I have put a lot of work into it.
My system specs are Core2Duo E8400 @ 3.4Ghz, 2x HD4870 512mb crossfire, 4gb (3 usable) DDR3
So, I'd say to other beginners, and probably some experienced modders also, to forget how good their own PC is, and hammer away at optimisation. Beautiful places are only beautiful the first couple of times you look at it, after that they are work places that people want to use, spend some time on making it run as smoothly as you can and strike a balance
Hey though Counciler, I'll be looking for particular feedback from you once it's released!