With the increase in the number and scale of data centers, more attention is being focused on their energy consumption and, above all, enabling a better balance between performance and power consumption. To address the trade-off problem, we propose a multi-server setup queueing model with a power-saving policy, where a server switches to a power-saving mode when it becomes idle. Upon the arrival of a job, a server will be set up to return to normal mode and process the job in the power-saving mode simultaneously. Under a Markovian setting, we obtain the steady-state probabilities and system performance. The analytical results allow us to evaluate the trade-off between performance and energy using an existing speed-based cost function. This paper presents several numerical experiments on performance evaluation and derives optimal values for speed parameters.