The Game Speed is the speed at which the game runs, and how fast time advances. Just like in real SimCity, Simburbia offers this mechanic.
In the Main Menu, there is an option to change the game speed. There are three speeds: Turtle, Llama, and Cheetah. Turtle is the slowest, Llama is the normal speed, and Cheetah is the fastest. The faster the speed is, the quicker you will get your monthly Income and events will happen in the city, and vice versa.