My argument is that it is theoretically possible that in a declining market environment, the combined effect of HFT and other algorithmic traders (I apologize for conflating and using crude terms, but my vocabulary is admittedly imprecise and incomplete) could accelerate a market selloff and overwhelm the normal mechanisms that have evolved over time.<p>For example, one worry I have is that algorithms may crowd out non-silicon short sellers.  As much as short sellers are demonized as &quot;evil speculators&quot; in times of market stress, sometimes in a down market they are in fact the only buyers available and actually slow down declines when they cover.  I don&#x27;t have solid evidence that short sellers are crowded out by HFT... but I am trying to provide an example of a scenario where the &quot;macro&quot; effects could transcend the nuanced &quot;micro&quot; arguments about cost, efficiency, liquidity, etc.  There are numerous other plausible scenarios that fit this bill as well, and it is the uncertainty around them that I am trying to point out.<p>Of course, none of these things have to do with IEX or a time delay... maybe things like order stuffing or spoofing are related but even those things are not what I&#x27;m talking about.  Especially considering the dialog that has taken place since Flash Boys was released, I feel that both industry and regulators risk getting caught up in a cost&#x2F;benefit analysis of the small issues while ignoring potential systemic problems.<p>As an investor I don&#x27;t really have a huge dog in this fight... I&#x27;m not expecting any pension, and I don&#x27;t own any managed funds.  I generally use fundamental analysis to do long term investing in broad sectors using ETFs and a smattering of individual stocks.  I manage my own money and so I will have no one to blame but myself if things go wrong.  And frankly, if I lose $0.0002 on a trade by being frontrun in a dark pool it doesn&#x27;t change my life, so for me to argue for or against IEX is somewhat irrelevant.  However as a taxpayer I am concerned that public pension systems could very quickly find themselves vastly underfunded in a short period of time, and as a human being I worry about people who have worked their whole lives and just want to retire in peace.<p>I think that my concerns are not entirely disconnected from what the general public feels&#x2F;understands about &quot;modern markets&quot;, so while in most cases I probably have no basis to refute your specific arguments, I suppose I take exception to people with &quot;macro&quot; views such as mine being dismissed as unsophisticated proles.  Even if that is not what you were trying to do... Matt Levine definitely comes off like that in his article.