Clearing is just on the settlement side and doesn't impact anything. Where were your orders executing? Does the trading platform show you or is it hidden? Citadel will have some price improvement as they have alot. Sometimes brokers will reroute orders if you can obtain price improvement but the smaller ones never do that. I think FirstTrades fits in that category.
I re I really don't know where the orders are executing. All I do know is my limit orders have hit my price several times and I have yet to receive a fill on exit with Firstrade. They are ITM butterfly spreads on the NDX if that makes any difference.
I'm going to answer...But I may be wrong. The market makers "make" the market many times. They see your limit order coming and jump in front of you. They move the market to their advantage...It seems illegal, but they can (and do) this. They have tried to get congress to intervene, but it doesn't happen. The MMs do help make the market, but they are NOT your friend...
The SPX options trades are executed by CBOE ultimately. I have never seen them being executed on any other exchanges if that answers your question.
If you are getting the vast majority 99% executed on one exchange then there's issues with the Broker's order routing/best execution practices. That one exchange "wins" the vast majority is like a sports team winning 300 times in a row. Boggles the mind.