Honestly, I have confidence in my English vocabulary, at least for a non-native speaker, but have to say that this quiz was hard. I'd wager that even native speakers might have had problems, and that's without the limiting factor of 10 seconds.
As to how this quiz could have been done better. The questions were too verbose. Given that the participants are working under time constraints, you should keep the questions as short as possible. It would have been better to phrase the questions in the form: Antonym of ...?
As for a more general improvement, maybe look into the time per question. Maybe a system where you can adjust how much time the users have to answer a question, based on how hard the quiz is. For example, this quiz would have benefited a lot from additional 5-10 seconds , but the previous one was fine as is.