It is lovely to see the effect of "arbitrageurs" making markets "more efficient."As an academic I enjoyed reading the post as it provides a very nice example of a clean empirical test of how high frequency trading makes the comovements of two markets stronger. This is what we look for in academic papers, a clean test of a very simple theory that produces very credible and robust results.
But as much as I enjoyed reading the post, it also reminded me of how limited is the ability of academic research to help us understand phenomena that really matter. In this particular case, the analysis compares the price of the same security in two nearby markets (geographically but also linked by very fast communications). As communications and trade become faster and faster, price deviations between the two markets disappear in a shorter period of time. This is really nice to see but is this a big surprise? One would expect that at a minimum, very basic arbitrage opportunities do not exist in integrated financial markets. So it is reassuring that arbitrageurs help markets be more efficient but I am not sure I would go as far as saying that this is "lovely".
What would be more interesting (at least to me) is understand whether high frequency trading helps getting financial prices right. And by "right" I mean prices that are consistent with economic fundamentals, prices that do not generate volatile dynamics and bubble-type behavior. If we could prove that this is the case then I would find the result "lovely".
Antonio Fatás