The actors are the users of the system in a broadest sense. This includes end users, batch processes, or external systems, depending on the use case.

By using the system, the actors will generate work load. The properties of the actors include the load factor, that is how many users are involved, as well as the velocity, that is how fast user requests are processed. These properties influence the overall situation similar to all other subsystem's properties.

The actors themselves don't represent a performance issue, they simply use the application. That said, if the system's performance isn't met, the limiting constraint is not to be searched for within the actors; the actors and the load they generate are part of the circumstances the system has to deal with.