It simulates a computer entirely in software, which is expensive. This is different from for example Virtualbox which virtualizes a computer, and still executes most of the instructions directly on the processor.
Modern preemptive multitasking operating systems send the CPU to sleep when there's no work to be done. In DOS, the currently running app or game usually just used every available cycle (aside from interrupts handled by the BIOS or DOS) for its own purposes. I don't know if Dosbox has some kind of limiter in place, but if not, the game/app will eat every single cycle it can unless it's specifically designed to wait for the vsync interrupt.
