I heard that the transfert speed of bitcoin is very slow so we can't use it in real life to buy coffe, is it true ?
No, thats not true. The transacion speed of all Coins is the same. In the order of seconds.
What many are talking about are confirmations. But confirmations are a measurement of security. Simple speaking, when you have 1 confirmation in Bitcoin it means the mining power of 10 minutes says your transaction is confirmed. If you have a coin with 1 minute Blocktime 1 confirmations means the mining power of 1 minute says your transaction is confirmed
That means if you wait for 6 confirmations in a Bitcoin transaction, you should actually wait for 60 confirmations in the other coin and that only if the same amount of Hash Power would back it.
The only (very small advantage) is the 1. confirmation where it would already have the security of 1/10 of a Bitcoin confirmation.
But faster confirms also have disadvantages like more orphants which increase the cost of securing this network.
That's easy to misunderstand if you are new to crypto and altcoiners usually have no interest in clarifying this (most of the time they do not understand them self or do not want to).