None of these solved my problem but I attached my save file and if you have time please have a look at it and let me know if you found something.c1normal.zip
Had a run at it and managed to remove the lag.
Delete all you stockpiles (people on one side of the river can't access them thus making a lot of dead commands.
You had 30 breads on order and not enough wheat along with a few orders to build what I presume to be wall underground but the mason bench was at the other side of the river.
Basically all you have to do is remove all orders, finish the bridge and issue orders once you have enough raw materials to back most of them up.
Now I don't know how powerful your pc is, but I found that on my old pc I get a small lag if I dig to far down without clearing the bigger parts of the dungeons.
Have fun. You have a good map with lots of iron so you should be able to equip iron armour to all you townfolks pretty fast.