Simply put: slower.

A quantum computer can solve certain problems within a certain computational complexity class, which would fall in a different class on a classical computer!

Given a long enough amount of time, a classical computer can calculate everything a quantum computer can.

The next part in the tutorial series is out https://news.ycombinator.com/item?id=17637553

It is a quantum computer emulator. Regular binary computers can model quantum computers with a smallish number of q-bits (dozens) by the time we get to a quantum computer with hundreds of q-bits all the regular computers on the planet together will not be able to simulate it.

Every turing machine can emulate every other turing machine. Different turing machines are all equivalent in what they can compute, but they can computer things at different speeds.

Both binary computers and quantum computers are turing machines.

simulation/emulation