Not quite. With Betfair's in-play betting they have what is called an "in-play delay". Its a couple of seconds delay between you submitting a new bet and it actually going into the system. There is no delay for someone cancelling an existing bet in the market. The reason for this is to allow time for people to cancel existing orders in the market if something bizarre happens.

E.g you could have money to lay on a horse who is running last. Then suddenly the leader falls and brings everyone else down except your horse. Thousands of punters could try to jump on and snap up your lay for a "guaranteed win" but you get the chance to cancel it first. But you gotta be watching!