Beginning of my creature_standard.txt
creature_standard
[OBJECT:CREATURE]
[CREATURE:DWARF]
[DESCRIPTION:A short, sturdy creature fond of drink and industry.]
[NAME:dwarf:dwarves:dwarven]
[CASTE_NAME:dwarf:dwarves:dwarven]
[CREATURE_TILE:1][COLOR:3:0:0]
[SPEED:600]
Default speed is 900. I found 400 or so a bit too fast and it's twice the speed. It only takes them half the time to get things done.
I have found that in previous versions if your fortress gets huge and population gets like 150 then Dwarves start to eat and sleep before they finished a single task because of walking distances, making the game unplayable. Of course now there's burrows.
How insanely fast is [Speed:1] Does that mean everything gets carried out instantly? So you can pause the game, spend hours designating an insane fortress with towers walls and deep labyrinths, turn off pause and immediately it's all done, if you computer can handle it?
I recommend speeds between 400 and 900. Anything less is too blunt cheating. Elves have default speed of 700. If you don't increase the speed your fortress may get to a grinding halt when it gets too big because of the pathfinding and the inefficient way dwarves carry out stuff.
Agility affects speed. But if you change agility you also change everything else influenced by agility.