A key factor influencing sound quality in open-fit digital hearing aids is the processing delay. So far, the delay limit needed for ensuring optimal (rather than tolerable) sound quality has not been established. Using a realistic hearing aid simulator, the current study investigated the relationship between preferred sound quality and five processing delays ranging from 0.5 to 10 ms in listeners with normal and impaired hearing. The listeners with normal hearing showed a strong preference for the shortest delay. For the listeners with impaired hearing, participants with mild hearing losses below 2 kHz also preferred the shortest delay.