This paper investigates the problem of prescribed performance tracking control for a class of disturbed nonlinear systems based on an event-triggered strategy. A nonlinear disturbance observer is introduced to estimate unknown disturbances in the system, while a prescribed performance function with a time-varying decay rate is incorporated into the control design. By utilizing command filtering and an iterative design algorithm, an event-triggered prescribed performance tracking controller is constructed. This controller not only ensures that the tracking error consistently meets the prescribed performance criteria but also exhibits excellent disturbance rejection capabilities. Compared to traditional performance functions with constant decay rates, the performance function with a time-varying decay rate effectively prevents excessive control energy at the initial stage. Finally, the effectiveness of the proposed method is verified through a numerical simulation.