When the process is working normally, process operating performance optimality assessment makes a further judgment on how well the process is running currently. Sustained optimal process operating performance guarantees the greatest economic benefits. However, the operating performance may deteriorate from the optimal status due to the production condition shifts. It is crucial to accomplish operating performance optimality assessment strategies for a deeper understanding of the process status. A Dynamic Causality Diagram (DCD) explicitly and intuitively describes the causal correlations between variables. However, an actual process has both quantitative and qualitative measurements, which makes the conventional DCD imprecise or even invalid. To handle the above problems, a modified DCD is proposed to assess a complicated process with coexistence of quantitative and qualitative information. An operating performance optimality assessing strategy based on the proposed DCD is then established. The proposed method is successfully applied to a gold hydrometallurgy process, and the operating performance optimality assessment result is satisfied.