I am jumping in a little late, but know what you mean. Duplicating a quiz does take longer than it should, but it seldom takes my (small) Moodle (with lots of questions) more than 10 minutes.
I am not sure what you are trying to accomplish. Are you duplicating to reuse the exact quiz somewhere else in your course, or are you duplicating to keep your quiz settings, while wanting to change questions?
What I most often do in a new course is to create a quiz with all of the settings set, but no questions. Then I duplicate this "shell" quiz, moving them into topics. Then I go into each quiz, make some title changes, and finally add questions. I hope that you understand what I am saying. Continue asking questions, if needed.