Advanced wireless services and applications are demanding lower latency and better reliability. In IEEE 802.11 networks, slicing abstractions at the Medium Access Control (MAC) layer can provide precise resource allocation and traffic isolation as a means to meet these performance requirements. In this paper, we propose a delay-aware approach for MAC management via airtime-based network slicing and user association using Multi-Criteria Decision Analysis (MCDA) in IEEE 802.11 Software-Defined Radio Access Networks (SD-RANs). To enable such an approach, we leverage Software-Defined Networking (SDN) to monitor queueing delay statistics at the Access Points (APs). We evaluate our approach in a testbed with two APs, controlled by SD-RAN and SDN controllers with four Stations (STAs) served by both Quality of Service (QoS) and Best-Effort (BE) flows dynamically. We compare our approach to a state-of-the-art user association approach. Our results show that in addition to load balancing flows across APs, our approach enhances the QoS delivery at runtime using slicing.