A performance scaling device, a processor having the same, and a performance scaling method thereof are provided. The performance scaling device includes an adaptive voltage scaling unit, a latency prediction unit, and a variable-latency datapath. The adaptive voltage scaling unit generates a plurality of operation voltages and transmits the operation voltages to the variable-latency datapath. The variable-latency datapath operates with different latencies according to the operation voltages and generates an operation latency. The latency prediction unit receives the operation latency and a system latency tolerance and generates a voltage scaling signal for the adaptive voltage scaling unit according to the operation latency and the system latency tolerance. The adaptive voltage scaling unit outputs and scales the operation voltages thereof according to the voltage scaling signal.