Why is there a delay when placing bets?

With in-play markets, Betfair introduce a delay of up to 9 seconds depending upon the specific market. We do not introduce this delay and there is no way around the delay.

From the Betfair website:

Why do you have a delay on placing bets on a market that is ‘in-play’?

‘In-play’  markets usually carry a time delay varying from 1-9 seconds. This delay is in place to allow customers to cancel unmatched orders on the system when there is a change in market conditions. This delay protects both backers and layers and leads to greater liquidity.