macrocephalopod

Document means to provide evidence, not make a simple assertion. Provide the tick data and the trades that *show* it.

i would never betray my clients trust by posting their order flow

I am pro hft in general but there is definitely an argument that most of the px improvement that retail sees is not ‘real’ — in the sense that by moving retail flow off exchange, you leave the more toxic flow (from prop shops, hedge funds etc) behind, so exchange market makers...

need to post wider spreads. Since px improvement is measured against exchange quotes, it is mechanically an ‘improvement’ because its being measured against a price that has been made worse by the very fact of moving the retail flow off exchange.

I think I explained that v badly but is there anything wrong with the argument? It seems like you need to compare vs the counter factual where everything executes on exchange, not just compare retail executions against exchange quotes.

Yes this is a very good argument! Totally plausible to me that retail orders do get better execution from SIs than they would if they were on an exchange, precisely because separating out their flow allows price discrimination.