The speed is the time the computer waits between two generations. There are five speeds you can choose using the Speed menu: Fastest is 2 tenth of a second between two generations, Fast is 4 tenth of a second, Medium is 6 tenth of a second, Slow is 9 tenth of a second and Slowest is 12 tenth of a second.
The corresponding keyboard shortcuts are:
Fastest: key 1
Fast: key 2
Medium: key 3
Slow: key 4
Slowest: key 5