Once you have initiated a withdrawal, the transaction is being automatically created and sent to the blockchain. Please allow some time for it to get confirmed by miners. We are not able to affect the speed of this process as it depends on the current network load.
If you change your mind and want to cancel the withdraw, you may cancel the transaction while it is not sent to the blockchain yet.
This can be done on "Deposit/Withdraw" section
In case you do not see this option, it means that the transaction has been already sent to the blockchain and is currently being processed.