Number of items have little to none impact on FPS.
It's pathing that counts. If you are playing latest version, tattered clothes (if replaced) are removed from dorf's possession and are stored in "finished goods" stockpile. And despite the fact that they will try to claim the best available quality it does not give a specific good thought, it's only the having tattered or having none that gives bad thoughts.
About the number of items not influencing FPS< not too sure. That's a 2012 thread, not sure if that part hasn't been debunked.
Also, if a dwarf replaces clothing with a better piece, the old piece does NOT go to a stockpile. The dwarf retains ownership of it, and he will eventually take a 'store personal item' job and store it in a personal room if he has one. This eventually leads to rooms / cabinets full of clothes.
I posted a thread about this last week, will find link and edit in a minute. EDIT: http://www.bay12forums.com/smf/index.php?topic=156377.0
Also, claiming a new piece of clothing does give a good thought if it's of decent quality (maybe only masterpiece and artefact, perhaps exceptional too). On a side note, how to keep your full time military satisfied with acquisition needs and putting on a splendid item thoughts: Every once in a while, make them strip (set them to wear nothing, then make them put their armor back on. Works wonders with masterpiece armor, don't know if it works with lesser quality (my dwarves never wear anything but masterpiece armor, armorsmiths are easy enough to train up)
1.
Maybe I am mistaken here about stockpiling, but I assumed it was the case as: there were no spare ones lying around and my FG stockpiles were full of tattered clothes. Of course,
maybe it was just decaying, or
maybe something in DFhack that is on by default.
2. Claiming piece of closing does not give SPECIFIC thought, please note the word. There is no good thought related to clothing, there is one for any high-value item that ends up in dwarf's possesion whatever that is.
And most importantly
3. Quick DFHack test about items semi-confirmed FPS claim. Spawned 1k, 10k and eventually 88k more items, with no significant hit to FPS, it still fluctuated within 6 range in same numbers. Then making a stockpile and have dem dorfs haul THAT did drop FPS significantly. Of course there is a place for more testing with cutting pathing access to already stored and unstored items, temperature changes and making them wet / burning to see how much it affects, I guess containers too, but I can't play for extended periods of time ATM.