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I ran a 3-qubit measurement circuit on QASM simulator as well as on IBM Q Melbourne, Santiago and Vigo machines. The running time on the simulator is in few milliseconds (400-800) while in the case of all three quantum machines the running time is in seconds (4-13).

Why such a large difference as it does not give a clear picture of simulators' accuracy in comparison to realized of quantum hardware?

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    $\begingroup$ I guess that real quantum processor has to be "calibrated" between shots, while simulator is simply an algorithm run in cycle. As the calibration takes some time, the processing time on a quantum processor is longer. $\endgroup$
    – Martin Vesely
    Sep 16, 2020 at 7:43
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    $\begingroup$ There is also a considerably larger queue for the physical systems $\endgroup$
    – JSdJ
    Sep 16, 2020 at 9:31

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A simulator executes just an algorithm on a classical hardware as Martin said. On a real quantum hardware, your circuit is calibrated before actually being executed. In addition, there are other tasks like loading pulses into waveform generator, qubits relaxation...which take time and explain the difference.

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