How to alter the speed in Dwarf Fortress?

Short answer: You can't.

Long answer: Get a faster computer. I've noticed that when I first start a fortress that it runs very quickly (70-80 FPS) but once I have 50 dwarves running around the FPS will drop to around 20-30 and game speed is tied to FPS.

Edit: C. Ross is correct. Dwarf Fortress is only single-threaded so the speed depends entirely on the speed of whatever core is running DF.

Yes, you can try.

Lowering [FPS_CAP:X] in the data\init\init.ini file can lower the speed of the game. If you're running at your FPS cap, increasing it can increase the speed of the game. Normally, however, the cap is not reached and your processor speed is the bottleneck.

Lowering [G_FPS_CAP:X] can actually increase the speed slightly, if you're not running at your FPS cap.

Lastly, I will add that the single most important thing I have personally found that decreases speed is your embark area. If it's large, your game will start to crawl after you get a larger number of dwarfs. And now that magma can now be found in every embark square, I usually stick to areas of 3x3, and my FPS hums along at a nice 120.

Above answers are answers to question "Can I make DF run faster", which is a bit different question from "How can one increase/decrease the speed of Dwarf Fortress?"

You can manipulate simulation time by changing FPS limit. By default, you can do that using Alt+equals, Alt-mins key binding. The upper speed limit is bound to your CPU processing capabilities.