[SEJeff]

I'm familar with NBBO, my previous employer (Virtu Financial) and current employer (another large electronic trading firm) are very well known HFT firms but I'm in tech, not the trading side. That doesn't mean the NBBO doesn't change before your order can fully execute? Is it fair to assume you've never heard of RBC's Thor[1] and aren't super familar with maker-taker rebates and the scandal behind them? The NBBO and the proliferation of trading exchanges is in fact what allows this to be possible legally. Like many regulations, it became a tool to perform the types of things it was designed to prevent. This article[2] has an interesting take on the NBBO wrt the SIP that is used. Many firms can calculate their own information faster than the NBBO updates. Note that this isn't the kind of trading (speed only) that my current employer does at all, which is why I sleep well at night.

[1] https://en.wikipedia.org/wiki/THOR_(trading_platform)

[2] http://www.nanex.net/aqck2/HowSlowIsTheNBBO_%20A_Comparison_...