It has been shown in previous work that asynchronous event-triggered mechanisms can reduce measurement transmissions of control systems' implementations. In this paper we study the relation between minimum bandwidth of the feedback channels and the performance of linear systems. By using the S-procedure, the parameters in an asynchronous event-triggered implementation are optimized. Threshold update delays are considered, and two methods are presented to guarantee a certain performance under such delays. With the presented results, the implementations can be tuned to reduce the amount of communication rate while guaranteeing pre-specified performance levels. Simulation results are presented illustrating the applicability of this optimal asynchronous event-triggered implementation.