Hey, I just read your thread, Kohaku! I'm eagerly awaiting more tests on this, because it doesn't look like anyone else is doing it currently. That variety causes lag is an assumption on my part that I made based on what others have said (some comments in the DF masterwork thread as mentioned earlier), and good old-fashioned hunches and intuition. The lag produced by caravans is pretty significant even with forts that have a ton of stone lying around, which makes me think that the variety of goods they bring and their clothing are to blame. Gods, the clothing! There's a bazillion different types of plant fibers and leather that can make up many different types of clothing, and that's not even taking armor into account.
Something else that bears mentioning, is when I reclaimed my two-year old fort due to a tantrum spiral. I noticed a permanent drop in FPS, bringing me down to 50, from 80-100, which is nothing to sneeze at. I don't seem to remember the frame rate being that low before reclaiming and I'm getting better numbers with a similar number of dwaves now. And this was just with my starting seven dwarves.
So, what I did was use my book keeper to mark all the crap strewn around the wilderness (there was a LOT of clothing and equipment from the previous fifty-dwarf tantrum spiral) for dumping and destroyed it all with DF Hack. I estimate about 600-700 items were removed in total, and the change was immediately noticeable, I gained about 30 FPS right off the bat. This is a little puzzling since my fort has produced about 2000 units of stone plus other goods, yet it didn't seem to have such a drastic effect on framerate.
I think this is either an issue of variety, or perhaps the items being out of stockpiles and strewn in far-off regions of the map. I can't say which, but you would certainly be the one to test this if you felt like doing it; the conditions this all happened in were far from a controlled environment and I can't fully remember the exact numbers and details.