Mixed-critical systems consist of applications with different criticality. In these systems, different confidence levels of Worst-Case Execution Time (WCET) estimations are used. Dual criticality systems use a less pessimistic, but with lower level of assurance, WCET estimation, and a safe, but pessimistic, WCET estimation. Initially, both high and low criticality tasks are executed. When a high criticality task exceeds its less pessimistic WCET, the system switches mode and low criticality tasks are usually dropped, reducing the overall system Quality of Service (QoS). To postpone mode switch, and thus, improve QoS, existing approaches explore the slack, created dynamically, when the actual execution of a task is faster than its WCET. However, existing approaches observe this slack only after the task has finished execution. To enhance dynamic slack exploitation, we propose a fine-grained approach that is able to expose the slack during the progress of a task, and safely uses it to postpone mode switch. The evaluation results show that the proposed approach has lower cost and achieves significant improvements in avoiding mode-switch, compared to existing approaches.