I was trying to take the online quiz for a course but the system is moving very slowly and I can't save questions or move to the next one.

Try logging out and coming back in. If you are still unable to access the quiz, it is possible that something happened with your internet connection that would prevent you from completing the quiz. If you are unable to get in to your quiz, your instructor will need to either reset your attempt or grade what is there. Let your instructor know of the problem as soon as possible.