With discrete GPUs, using system RAM is slow not due to mem bandwidth, but due to PCIe bandwidth, which is the bottleneck.

For example, 16x PCIe 4.0: 256 Gb/s, 16x PCIe 5.0: 512 Gb/s, while 2x DDR5-6400 DIMMs: 819 Gb/s. The actual throughput is lower for both PCIe and DDR5, due to communication overhead.

On server/workstation motherboards which may have 4, 8 or 12 DIMMs instead of 2, the ratio between memory bandwidth and PCIe bandwidth becomes proportionally higher, so the memory throughput achievable by the GPU becomes a very small fraction of the system memory bandwidth.