instead of changing your limit order into a market order just change the limit price. So if buying set the limit above the market and below the market for a sell that way you'll be executed(!) at the market price straight away.
If you are trading Globex and you enter a market order then IB has to convert the order into a limit order prior to sending the order to Globex. This is where there is a very slight delay. In a fast moving market this will make a difference.
If you enter a Limit or Stop Limit order the order is immediatly routed to Globex.
FYI: The GLOBEX matching engine doesn't accept stop orders! Your broker probably will take them for GLOBEX markets! How is this? The broker keeps your 'stop order' on their server (big computer) until the price is reached. Your brokers software then sends your stop order to the GLOBEX engine as a market order.
If your broker has built his software around PatSystems then your stop will actually be held on YOUR PC not a central server that you'd hope was more stable than a/your PC. If you turn your PC off then bang goes your stop order!
As for eCBOT being built around LiffeConnect, orders will be filled in priority of size, not time stamp.
This is my understanding of some odds n sods of getting orders filed that I hope helps. Others may know different???