Are fills in the the demo platform realistic? Do they represent actual fills from live markets?

Quick answer: for the most part, yes, and no, they do not represent actual fills.  Though live orders are virtually instant, your experience can vary due to a number of factors.  This includes quality of internet, client's computer specifications, location of exchanges, liquidity, to name a few, may affect order execution. With regard to demo orders, fills cannot not be directly compared to how you may be filled in the live markets. The purpose of the demo software is to become acquainted with our features. In the live markets, orders are filled First In First Out (FIFO), depending on exchange rules. Even though the demo filling engine does take order placement into consideration, the market depth may be fictitious, ie. the number of orders sitting at a given price level may not match the live order book. Also, where you sit in that filling queue may not be representative of where you sit at the actual exchange. With that said, though our demo fills may seem realistic, the time you may receive a fill may not be exactly when you would be filled in the live market, if even at all.  We simply match demo orders to our tick feed, using proprietary order placement.  There is no way to accurately match demo orders to live orders, please keep this in mind when using the simulator.