Small-message aggregation is critical for scaling irregular, communication intensive applications in high-performance computing. In this paper, contrary to conventional wisdom, we present the first systematic study showing that memory contention, not network bandwidth, is the dominant bottleneck in message aggregation runtimes. Using the state-of-the-art conveyors library as our reference implementation, we conducted extensive experiments on HPC systems featuring Slingshot 11 and InfiniBand interconnects, scaling to 16k cores (256 nodes) and processing 10s–100s GB of data. Our measurements reveal that interference between user data and aggregation buffers drives LLC miss rates to 77%, inflating memory costs by 2–3× over the algorithmic baseline. Consequently, we advocate for dedicated near-memory subsystems to improve the scalability and performance of message aggregation runtimes. This paper also demonstrates up to an order of magnitude higher latency for conveyor termination compared to a traditional HPC barrier, and it examines the impact of communication context isolation and the critical challenge of programmability.