Despite the growing use of slag within concrete mixes, performance uncertainty persists. While the literature highlights that slag inclusion can reduce slump loss over time, these studies have largely focused on the self-compacting range [1–5]. Furthermore, there is currently inadequate understanding of performance in rheological terms. This study aims to reduce uncertainty by assessing a range of mixes with slag contents up to 50%. The results show that the water-binder and aggregate-binder ratios influence slump loss, with higher ratios leading to lower total slump loss. Additionally, the water-binder and aggregate-binder ratios influence the effect of slag inclusion on slump loss, with small reductions in total slump loss seen for intermediate ratios and larger benefits associated with higher ratios. In contrast to slump loss, the static yield stress was found to increase with higher water-binder and aggregate-binder ratios, with greater growth observed for mixes with slag. This contradiction was attributed to deficiencies in total slump height as a proxy measure of performance. Instead, the unyielded height, derived from the slump profile, offered highly accurate predictions of the yield stress that matched the observed growth in rheometer derived yield stress. These results indicate that rheology provides more informed insights into fresh state performance than the slump loss parameter.