Cascade control structures are widely implemented in industries, due to the fact that they increase the performance of single-loop control. Nevertheless, their implementation requires a higher initial investment. Because of this, it is of great importance to be able to quantify their relative performance improvement. Very few studies handle this subject, and they do not obtain results which directly establish this quantification on a general basis. Therefore, the objective of this project is to conduct a study with help of computational tools, in order to determine the relative performance improvement of a cascade control structure with respect to a single-loop structure, for setpoint tracking and disturbance rejection. An overdamped second-order-plus-dead-time process, separable in two first-order-plus-dead-time subprocesses, was considered. The controllers were implemented using a PID algorithm. Simulations were conducted to obtain the ratio of the IAE performance indexes of each control structure, as a function of the time constant ratio, for different process parameters. The controllers were tuned by means of optimization routines, in order to guarantee that they provided the best IAE index. Finally, two general equations were obtained, which quantify the relative performance improvement of a cascade control structure with respect to a single-loop structure, as a function of the process parameters, for set point tracking and disturbance rejection.