It's true that Linux kernel is the throughput bottleneck. Unfortunately, the optimizations described above aren't sufficient to get within even 10% of hardware bandwidth.
Even if the swap system overhead drops to just a data copy, the memory management layer prevents swap from scaling to higher bandwidths. The issue is not data movement; it is in the page unmapping step (which requires expensive TLB shootdowns). Larger kernel changes are required.
My group wrote a paper on this: https://dl.acm.org/doi/10.1145/3731569.3764842
Linux's swap system is undergoing some large refactors lately. Hopefully some insights either from our work or Hermit (NSDI '23) can make it in to the mainline. I think Hermit's `rmap` optimization in particular is a candidate for upstream use.