One of the authors here: What I was going for with the title is that singular read/write switching (the before case) is very slow (for packet sized work), and b

One of the authors here: What I was going for with the title is that singular read/write switching (the before case) is very slow (for packet sized work), and batching (~ >=64kb) is much faster - it's about amortizing the cost of the transition, as you rightly point out. That's the point the title is making - some interfaces do not provide the ability to amortize that cost, others do!