Yes that's what i thought too - The quantum computer might be able to process billions of transactions a second, but it could easy have a bottle neck placed on it if the system its trying to login to can only have a small amount of login attempts done system wide.

Remember too, the defense systems will also be running on quantum computers - So it may be compared now trying to using a desktop to hack another desktop.. that may also give the quantum computers alot less edge when trying to break into systems.