In general, characteristics of classical control theory and properties of real-time scheduling algorithms may cause unexpected control system responses in the implementation of real-time computer-controlled systems. Revising realtime scheduling properties, we analyse which are the main timing problems that current scheduling algorithms may introduce in the execution of control loops. Next, we categorise those timing problems, that is, jitters on task-instances executions, in a control context. Afterwards, we show, by simulations, different types of control system performance degradation that these jitters may cause. Finally, we propose possible solutions that solves this degradation, based on irregular sampling discrete-time system models with varying time delays.