Thanks for your replies!
I was thinking more along the lines of daytrading with up to a few thousand shares, and sending direct routed orders e.g. to ISLAND to buy at the ask and sell at the bid (if ISLAND is not the one with best price, they route it further if I understand correctly).
So if I understand correctly, the pro programs would actually take a SMART routed order, find the best venue, and get the order out faster than TWS will deliver the order I described above to ISLAND? Curious which time frames we are talking here. Would be interesting to see some actually measured comparisons, between time from submission to execution, and resulting slippage (on average of course).
This information might be a huge selling point for the pro programs, surprised they do not publish such studies for advertisement. (Or maybe they do?)