Thanks for mentionning the items, despite i use quantum stockpile i didn't thought it would be that huge before doing the experience myself :
i haven't got my FPS display, i only judge the game by how fast it's playing.
And on my 6 years old current fort, i have 100 dwarves (max cap i setup) on my 2x2 embark, and it's fortunately very playable i don't have any slowdown yet (due to the 2x2 embark and 100 max popi guess) but it's noticably and obviously not running as fast as it used to be in the 1st year.
So i decided to do the test, i pressed d - b - d and set my quantum stockpile for dump , then with dfhack i typed
autodump destroy
5000 items were destroyed (wow so much and i didn't even built much craft).
After i unpaused, the difference in speed is -very- noticable, that's quite impressive for me to see 100 dwarves running around that fast, as fast as what i could see on the 1st year when there was not much things on the map.
Good i had made a copy of that save as for the experience i destroyed all the weapons and armor i had forged on this
This is eye opener actually, i always thought the pathfinding was the biggest culprit in performance (as shown by a dwarf getting stuck on a tree or a wall and getting a mood), but from that experience, it's looking like the items accumulation through the fortress years are the big player there.
Considering how very fast items are accumulating in a fort life (by example i got
2000 mussel shells on that small map in less than a year !) even when you're not having much industry running, this item huge impact on performance is definitively something Toady should take a serious look to considering everyone will have to face the fps death problem if they play a "normal" (as in not making sacrifice of gameplay part to run without much performance hit) fortress more than only a few years.
edit : turned on the FPS display to check
before the eradication of all the stockpiled item in my quantum stockpile fps are going between 85 and 90 (as i said due to small 2x2 and only 100 pop, it's very playable) , after autodump destroy them all fps = 100 (probably because i'm at the cap there, as i didn't edited)
Though i use GPS=15 so it may (or not) have an impact, didn't tested much around that.
But when it comes to the dwarves movement , there is a definitive difference in speed.
I'll have to check my older saves, i had a much longer lived fort (more than 25 years), with a larger embark and more than 150 dwarves that was running slow, the experience would be more interesting with destroying all items.