Policy based Network Management

Friends,

We are planning to try with Policy Servers on our WAN for QoS. In the Policy framework, in which the policy server provides policy decisions to its netwrok device clients, is getting very popular. In this context, can anyone tell me how much time does an avarage policy server take to provide decisions to a device. (Is it in the order of millisec? or is in the order of Sec?) I am afraid the policyserver itself may hamper the performance of the devices.

Any hints/clues/cautions in this would be greatly appreciated?.