The Securities and Exchange Commission acting chairwoman signaled support for wholesale review of a practice that funnels many small investors’ stock orders to be filled by high-speed trading firms.The system, known as payment for order flow, is decades old but has generated greater scrutiny as more individual investors trade on brokerage apps operated by companies like Robinhood Markets Inc. Online brokers such as Robinhood make money by selling customers’ orders such as Citadel Securities and Susquehanna International Group LLP, which trade with them.In a letter made public Tuesday, SEC Acting Chairwoman Allison Herren Lee stated that regulators should examine such arrangements to ensure practices are completely disclosed and “consistent with best execution obligations.” That requirement considers whether clients get a better price than what is currently quoted in the market, the speed of execution and the possibility that their order will be filled. Ms. Lee’s letter, which was sent in response to questions from Sen. Elizabeth Warren (D., Mass.), didn’t state how or when the SEC might start a review of payment for order flow. An agency spokeswoman didn’t immediately return a message seeking comment.