The cascade control technology has been widely implemented through the industries. The focus of this paper is the assessment of a parallel cascade control system scheme against an achievable performance standard. Following the methodology of the univariate control loop performance, a procedure is derived based on the minimum variance and the Diophantine decomposition for the parallel cascade control system. The performance bound is computed using the minimum process variances which depend on the controllers. Besides, the achievable performance bound and the corresponding optimal parameters of the PID controller structure computed from the closed loop operating data are also proposed. It can assess the performance of the current given controller. If the cascade controllers are not operating at the achievable bound, the reduction of the output variability can be achieved based on the estimation of the controller parameters of the PID-achievable performance. The performance of the proposed method is illustrated through a simulation problem and a pilot scaled experiment.