Visual 6502 emulates the CPU not just at a logical level (performing equivalent instructions) but at a silicon level, simulating each transistor in the original chip at an electrical level as closely and as accurately as possible. This is far more intensive, as there's a whole bunch of state to maintain and thousands upon thousands of tiny components that need to remain in perfect sync. Of course the goal here is not to run the processor at anything resembling real-time speeds. Quite the opposite; the developers have created a marvelous way to slow down the components so that humans can observe each tiny step, which normally takes a fraction of a fraction of a fraction of a second on the physical chip.