Quantum computing can in fact only offer an improvement in speed, given what is understood about the fundamental underlying physics. There is no algorithm a quantum computer can run that the computer you are using to read this cannot: https://en.wikipedia.org/wiki/Church%E2%80%93Turing_thesis
Quantum computers represent a technological leap.
That doesn't strictly mean speed -- New technology is iterative and in a lot of cases start off weaker, but with greater potential.
Quantum computing can in fact only offer an improvement in speed, given what is understood about the fundamental underlying physics. There is no algorithm a quantum computer can run that the computer you are using to read this cannot: https://en.wikipedia.org/wiki/Church%E2%80%93Turing_thesis