Staggering will definitely help. 600 quizzes requested the same minute or in 10 batches of 60, in 10 min intervals are hugely different. There were even ideas of micro-staggering: protect the quiz by a long, easy to remember password - like a full sentence. The differing typing speeds and having to retype due to typos will stretch wave of requests. Well, need to combine with supervision, don't allow the candidates to sit on the F5-key waiting for the exam to appear!