That doesn't add up, because you're now exceeding the memory bandwidth of the memory controller. I.e. it would be faster to do everything, including the CPU only algorithms, in far away VRAM.
latency might be higher and latency usually dominates CPU side algorithms.
Where are you seeing 900 GB/s?
The 900 GB/s are a figure often cited for Hopper based SXM boards, it's the aggregate of 18 NVLink connections. So it's more of a many-to-many GPU-to-GPU bandwidth figure.