Free brokerages generally don’t execute orders as quickly in order to sell order flow which allows HFT firms to front-run your trade. If you’re a buy and hold investor, the fractions of a cent you’re losing don’t matter much over the long run but the issue is still there. Generally when you pay for something, it means that somebody is beholden to you if there’s issues. Robinhood et. al. are free because they suck.
No, they sell order flow because market makers know discount brokerage users are uninformed and thus less risky to buy and sell from. However, they actually give you better prices than the public market offers and they have to or else the SEC would fine them [0]. Your link is just about robinhoods shitty infra that couldn't handle their user load and has nothing to do with order flow.
I'm going to have to jump in here and say, no, HFT can and does not front-run your trade. There are reasons why your execution on RH is not as good as professionals, but it's not because of front-running.
Or through what mechanism do commissioned trades get lower prices?