The most surprising thing about the result is that there isn't an order-of-magnitude jump between SATA SSD and any sort of HDD, as you would expect with random read/write workloads typical of swap thrashing. Instead, the chart looks as if it is mostly measuring sequential read/write performance. HDDs have long been known to be faster on one end than the other in sequential benchmarks.
This could be an artifact of the particular kind of workload that the author used. Maybe it causes large numbers of adjacent blocks to be swapped in and out at the same time?
Author here. In all cases, most access is still RAM. The storage is only hit to stash or load overflowing pages.
I originally ran the benchmark with 1GB RAM instead of the final 2GB, but the start-of-disk test did not finish in the 9 hours I let it run. With 0GB, I don't doubt that you'd see the expected 1,000,000x latency difference between disk and DRAM.
This could be an artifact of the particular kind of workload that the author used. Maybe it causes large numbers of adjacent blocks to be swapped in and out at the same time?