The paper describes measurements and validation of a digital control system's performance (time) characteristic. Currently, in most cases, an industrial control (I&C) system is a distributed hybrid system combining hardware and software components that provide the necessary functionality. The I&C life cycle includes the stage of system tests to confirm its time characteristics concerning both speed (data processing time) in the system components and time (delay) of network data transfer between components. Various methods of time characteristic measurements are used. They depend on external conditions: experience and accepted practice and the properties of the measured parameters. In the work, we have researched the main statistical properties of the temporal characteristics of a typical I&C system for a Nuclear Power Plant (NPP). The measurements were carried out as with built-in system software as with an external testing agent. The results of both testings are discussed.