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However, this parameter's default value is "false". I wonder that "persistent == false" performs better in some cases. What kind of case is it ?

This is actually pretty well described on wiki/roscpp/Overview/Services - Persistent Connections:

ROS also allows for persistent connections to services. With a persistent connection, a client stays connected to a service. Otherwise, a client normally does a lookup and reconnects to a service each time. This potentially allows for a client to connect to a different node each time it does a service call, assuming the lookup returns a different node.

Persistent connections should be used carefully. They greatly improve performance for repeated requests, but they also make your client more fragile to service failures. Clients using persistent connections should implement their own reconnection logic in the event that the persistent connection fails.