When most of the elements are no-ops/unused, I don't think we should be making any assumptions about the performance at max capacity. Contiguous iteration is cheap. The author may call it the constant work principle but I can't agree.

In the context of HFT, since the author drew inspiration from the domain, there's also the issue of now having introduced new branches into the hot path. A lot of work goes into reducing branches and priming the predictor in advance of orders actually being placed. Granted, you could potentially be avoiding branches elsewhere as a byproduct but that's probably getting into the weeds and nitpicking the examples.