What is the reason for the exponential speed-up of quantum computers? [duplicate]

In quantum computers the following two effects should be seen:

1. If an operator acts on an arbitrary qubit $$Q_n$$ of a quantum system $$S$$ consisting of several qubits than we get a new quantum system $$S'$$ where the amplitudes of all qubits have changed.

2. If an operator acts on one qubit of a quantum system $$T$$ consisting of two entangled QuBits than both qubits are affected.

So which one of these is the reason for the exponential speed-up expected from quantum computers?