hi,
When a user takes a quiz then all the answers are saved in the local storage only, so when user clicks on submit quiz button then all the answers will be saved one by one in the database. So if there are many questions and many users are submitting quiz then yes it will take some time.
If clicking on next question takes much time, then please try using in course quiz. You can enable in-course quiz from the wp-admin > LMS-> settings > in-course quiz.