As your game world fills up with creatures, the pathing will take up more and more of your computer's processing power, more items, more history, more areas to path through etc. As you get more processing power used to do all those things, the game can only calculate so many Frames Per Second (FPS). On action games, anything less than about 50 FPS is too slow for me to feel comfortable, DF under 10 FPS is where I start to get bothered. There are no actual animations to watch, and no camera motions to make you feel motion. You'll often start a fort at 100 FPS, by year 3 it's down to about 25, and dropping hard unless properly controlled.
Some people call low FPS, or framerate drops, "lag", but lag is most often a term used for latency in connection in online games (not an issue in DF). Sometimes a program doesn't immediately register commands, which some also call lag, but I call unresponsive.
I understand your desire for largest world and longest history, but waiting all that time for world generation just to lose the fort to fluid dynamics in year 2, or your first magma forge, seems a waste. I've found that smaller embarks and lesser creature numbers most effective at controlling FPS, but the world sort of lives on now while you play, old large worlds can also affect loading and saving.
I'm not trying to steer you away from a single thing, the greatest thing about DF and it's community is nearly all of them encourage experimentation, doing your own thing, and playing how YOU have fun! They encourage attempts, and failures too (sometimes especially the failures)! If you try something no one else seems to have thought of, the community will jump to your thread encouraging you to post your results.
You will have some memorable early forts, and once you settle in, you will start making connections that will turn the game into a story of your fort facing adversity. These types of things are easier in a smaller fort anyway.
My suggestions were just an attempt to ease your way off the cliff of DF, but forge your own path and be the stronger for it!
EDIT: Ninja'd with great info. Thanks Dirst!