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 current network load.
If you happen to change your mind to withdraw, you may cancel the transaction while it is not sent to blockchain yet (i.e., does not have a transaction hash).
Should your withdrawal have status "Pending" without hash within a certain period of time and you don't want to wait, you are free to cancel it.
In case you do not see this option on the Payment History page, it means that the transaction has been already sent to the blockchain and is currently being processed.