Keldor has been taking our traders lead and is snoozing his way through his year.

The only things I can think of for speed are:
1) The carp - there are always quite a few of them in the river, I doubt they are the main cause but they won't be helping!
2) Pathfinding/object number - The underground bits of the fortress are rather confused and cluttered in places. I've read numbers of stockpiled items can effect FPS as well, and we have a vast stockpile of food - it was around 14000 when I last played. I've no idea if it's true but I could see how that might cause some slowdown, the game having to keep track of that much food (along with all the other goods) and for dwarves to decide what theu're eating, drag it down to the dining rooms etc. Might be an idea to turn all the fields to fallow for a year or three and see if that helps?
But as Jools says it's ok on the whole as it's still more than playable and it gives plenty of time for contemplation of new projects - and as the OP says you don't have to do a whole year if you don't have time, just do what you can.