Software control systems may be subject to high interference caused by concurrency and resource sharing. Reducing delay and jitter in such systems is crucial for guaranteeing high performance and predictability. In this paper, we present a general approach for reducing delay and jitter by acting on task relative deadlines. The method allows the user to specify a deadline reduction factor for each task to better exploit the available slack according to specific jitter sensitivity. Experimental results confirm the effectiveness and the generality of the proposed approach with respect to other methods available in the literature.