Bitcoin transaction is fast because nothing impossible in technology. If someone gives you a bitcoin, you can easily convert it into something. In bitcoin transactions, it needs some of our real information to verify our identity.
Yes surely it is fast but it depends on the miners fee you got because when you are going to send some bitcoins and the miners you paid is not that acceptable.
Then the miners will put your transaction into the last queue and they will prioritize those transactions that has acceptable and paid miners fee.
So that's the reason why sometimes our transactions are slow.
There are really times that the transactions are slow even if you choose to pay a bigger fee. But so far, it's satisfying to me.