It's not that they can execute a computation faster in the traditional sense. It is that you can exploit the quantum behaviour in some types of computation (e.g. factoring primes) to make the answer more probable, and thus more likely to be "observed", when you read the state of the quantum computer and thus make it easy to locate the correct answer compared to traditional algorithms.
PBS Infinite Series has several videos on quantum computing and Shor's algorithm: 1. https://www.youtube.com/watch?v=IrbJYsep45E "The Mathematics of Quantum Computers" 2. https://www.youtube.com/watch?v=12Q3Mrh03Gk "How to Break Cryptography" 3. https://www.youtube.com/watch?v=wUwZZaI5u0c "Hacking at Quantum Speed with Shor's Algorthm"