Mission critical applications in domains such as Industry 4.0, autonomous vehicles or Smart Grids are increasingly dependent on flexible, yet highly reliable communication systems. In this context, Fifth Generation of mobile Communication Networks (5G) promises to support mixed-criticality applications on a single unified physical communication network. This is achieved by a novel approach known as network slicing, that promises to fulfil diverging requirements while providing strict separation between network tenants. We focus in this work on hard performance guarantees by formalizing an analytical method for bounding response times in mixed-criticality 5G network slicing. We reduce pessimism considering models on workload variations.