The most recent cellular generation, 5G, is being deployed on a large scale globally. The capabilities of 5G surpass all previous generations of cellular networks and support many new services compared to 4G. Despite this, at the same time, preparations for 6G have begun since user demands and technical development continuously push the boundaries of what is possible. Demands come not only from users. Also, society sets requirements, e.g., sustainability, coverage, and privacy. To support the necessary features in the network needed to meet the requirements, a new generation of the architecture is needed; one based on the most forward-looking design principles together with trends in networks, use cases, and whatnot. To show that the proposed new features will allow the future network to meet the set requirements, key performance indicators (KPIs) have to be defined. In this paper, we present six of the KPIs that the European 6G flagship project Hexa-X has identified as the fundamental ones to measure the most important aspects of a new 6G architecture.