Is this for sure? Do "lots of items in the game world" really cause an FPS hit?
Even more than you could think.
Basically, there are two main things that heavily affect the FPS in every cases, plus some in certain circumstances:
1. Path finding: the more tiles there are that can be accessible, the more it will cost to find path. Also, the more living entities there are on the map, the more it will affect it too.
Basically, that is one of the reason why people use ways to close access to big useless zones, physically or not (burrows, d-o menu, doors and bridges...)
2. Clutter: Every item is tracked, and every time a dwarf needs to do anything with an item (or thieves, or anyone else), it will do a lot of checks on the present items. Having a lots of stones laying around, a ton of crafts you never sell, stray ammo from fights, tons of clothes... Every item will affect FPS, unnoticeable, but large amount of them will have a big impact. That is the main reason for some DF Hack tools that destroy, remove owned items etc. Owned items are even worse, as the dwarves will check a lot of things about what they own. Clothes and items that deteriorate also do checks to know their own condition at this point, to know if they are to be broken or not, on fire, having water on them, blood, everything.
Optional 1: Fluid movements. this one is a really big one, and the main reason you might want to skip the waterfall mist generator. Moving fluids, meaning every fluid that you made move, for one reason or an other, will do a lot of checks every frame, so a big tank filling up, waterfalls, pump stacks, and every liquid that is not naturally there will hurt your fps very badly.
Optional 2: Temperature. Mostly heat from fire and lava. Every time you move lava or fire, every affected and adjacent tiles will check for temperature, and calculate whether or not it will affect it. Magma pumps can cause a big pike of lag, as well as flooding with lava, but all heated tiles and items will also affect the FPS. Disabling temperatures fixes this, but it will not warn you about magma being near (the hot stones), so it's a double-edged sword. Plus, if temperatures is off, nobody will be hurt by fire or magma, at most they will drown in it, so no magma trap either. (not sure about magma industry, never put it off).
Basically, that is it, but most people add population, but it is in fact due to the two first reasons, a dwarf by itself won't cause much lag, it's interactions and moves will cause it.
Hope it clears the matter a bit.
Note: It all comes from !!Science!! made by people on this forum, as well as a bunch of tests I did myself, in no case can it be regarded as absolute truth.
