Architecting Internet of Things (IoT) systems is very challenging due to the heterogeneity of connected objects and devices, and their dynamic variabilities such as mobility and availability. The complexity of this scenario is exacerbated when considering Quality-of-Service (QoS) constraints. Indeed, reasoning about multiple quality attributes, e.g., power consumption and response time, makes the management of IoT systems even more difficult since it is necessary to jointly evaluate multiple system characteristics. The focus of this paper is on modelling and analysing QoS-related characteristics in IoT architectures. To this end, we leverage on the concept of Emergent Architectures (EAs), i.e., a set of things temporarily cooperating to achieve a given goal, by intertwining EAs with QoS-related constraints. Our approach provides the automated formation of the most suitable EAs by means of a QoS-based optimisation problem. We developed an IoT case study and experimental results demonstrate the effectiveness of the proposed approach.