Watching Thomas Peterffy’s video message (link in first post) a couple of times, the following crossed my head:
— Could it be that IB is overly aggressive with price capping (above and beyond what is required by exchanges) in a misguided attempt to reduce those orders’ negative impact on the self-reported „monthly execution stats“ cost figure, that seemingly has been elevated to final yardstick? —
In those stats, IB adds commission cost + market impact, and that is defined as trade price vs. VWAP. In other words, it seems IB books it's clients' daily profit of new positions (vs. VWAP) or PL of day trades against its commissions to arrive at this number.
Orders that are dis-allowed by the price capping mechanism are usually aggressive orders that, if executed, might make this stat look worse.