What exchange? Are you asking the execution on the exchange itself after the order has been generated, passed through the broker, an onto exchange? If the exchange is good, it might have a spec or metrics or even tell you. If the exchange sucks nothing prevents them from writing inefficient software
Fyi, when I send market orders to the CME, through IB, I'm getting an average of 0.25 secs, between the time the order is sent and the time I receive the fill. Almost never above 0.3 secs.
There might be a very small difference in the execution of these orders on the exchange computer, but as others have said this is a rounding error in the context of your total slippage. If I was programming an exchange computer (which I have never done!) the obvious logic would be to convert a market order into a limit order with some arbitrary offset, so I would imagine the market order is slightly slower, but we're talking like 3 to 4 lines of C and maybe a few dozen clock cycles at most.... This is guesswork on my part though. Rob