DOSBox emulates the CPU, the sound and graphic cards, and other peripherals of a PC, all at the same time. The speed of an emulated DOS application depends on how many instructions can be emulated, which is adjustable (number of cycles).
By default, DOSBox tries to figure out what the correct speed for your game or application is. Sometimes, your game runs too fast. Or it just runs too slow. You can see the speed set by DOSBox in the title bar of the application window.