What is the best way to determine the time it takes a service to return?

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Is there some ROS function that returns the execution duration of a ROS service?

Not that I know of (as in: this specifically).

Or is the easiest way to get the execution duration to ros::Time::now() before and after the service is called and then finding the difference?

That could certainly work.

Another approach would be to use a profiler, perhaps something like swri-robotics/swri_profiler (but there are of course more).