[olliej]

The importance of quantum computers is not the absolute clock speed, but rather the ability to reduce previously exponential problems into sub exponential complexity.

For example take the classic attacks on DLP -- instead of essentially manually factoring the keys we can create a program (Shor's algorithm, etc) that essentially treats the problem of factoring as the task of determining the base frequencies of the key when treating the public component as a wave (at least as I understand it).

On an ideal quantum computer that task should be polynomial complexity, although there's still some (hopefully small) probability of error, but basically you just repeat many times and use a classical computer to verify the solution.