Transactions can be instantaneous if they are zero-confirmation transactions, meaning that the merchant takes on the risk of accepting a transaction that hasnt yet been confirmed by the bitcoin blockchain. Or, they can take around 10 minutes if a merchant requires the transaction to be confirmed. That is far faster than any inter-bank transfer.
Some merchants will charge a fee for debit card transactions too, as they have to pay a swipe fee for fulfilling them. Bitcoin transaction fees are minimal, or in some cases free.
With bitcoin, you own the private key and the corresponding public key that makes up a bitcoin address. No one can take that away from you (unless you lose it yourself, or host it with a web-based wallet service that loses it for you).
Purchase from merchant compared to the inter-bank transfer is irrelevant. inter-bank transfers is not something normal person would be doing on daily basis. On the other hand shopping is something peoples day today activity. Nobody likes to wait for ten minutes at billing counter for payment confirmation. Already lines are malls or bazars are quite long and timeconsuming. At this stage purchase of amount with bitcoin at mall or grocery store is less convenient. Credit or debit cards are way faster than bitcoin when it comes to the shopping online or direct shopping.
It is true that debit or credit cards are less secure compared to the bitcoin technology. One way to improve this speedy transfer is to use higher transaction fees. However looking at current rate of bitcoin transaction rates have also gone berserk.