You have to be careful with "market orders". Depending on your routing, market orders may not even be accepted !
As an example ISLD only accepts Limit orders, but simulates market orders by placing a limit order above/below the market.
You also have to understand how your broker deals with the limitations (as just described), time-outs, etc. e.g. if you want to SELL, your software may try to either hit specific MM in turn until you're filled, leave it up to SuperSOES, etc. Make sure you understand all of this before trading "fast" markets.
If you need to "panic out or in", the fastest way I've found is to do an above/below order on ISLD. Some of my trading buddies swear by SuperSOES - I haven't used it much.