To those who doubt that item counts have an impact on framerate, I should think you might want to look at
my experiments with increasing caravan capacities by a couple orders of magnitude to see just how much impact item counts can have.
When the merchants started bringing in tens of thousands of items, the
instant those merchants were loaded into the game (before all of the wagons even entered the map, they were simply waiting to get on the map), it made an immediate and profound impact on my framerate, dropping it from 60 or so FPS down to about 12.
Subsequent atom-smashing of the trash raised my FPS back up to around 35, although by that time, 2 years had passed, and I had more fluid motion and population to drag down FPS.
One of the problems is that every single object in the game (aside from those in constructions) is apparently iterated through
every single frame as the game currently stands. This may be because of Temperature (this is just speculation on my part), which will check what state of matter every item should be in every frame based upon the ambient temperature, meaning that even if nothing is even looking for your 20,000 pieces of trash locked away in a place nobody is looking for or can even path to, then the game still checks that stuff against ambient temperature.
Pathfinding is the major killer in many forts because pathfinding is the one thing you can't possibly help having the computer do.
Fluid mechanics are brutal and obvious FPS killers, and were responsible for about 30 or so FPS loss in my last fort because I had a serious waterworks project. And I know that it was precisely the fluid mechanics that did it, because the presence of water in my resevoirs and piping created measurable 10 or more FPS drops which I could regain by flushing the system down into the bottomless pit.