For HFT, it's not that every second counts, is that every millisecond (or even lower) counts. You're competing with other, similar algorithms for picking up opportunities. The assumption is that you're not capital constrained, you (or the competitors) can immediately exploit all the volume of such an opportunity, the deals you submit shift the prices so that it disappears.

This means that whoever is not the first to take that opportunity doesn't get it, and if you're reliably a millisecond slower than a competitor then you might as well not even try.