One of my major concerns during quizzes is always "click count" because clicking repeatedly accumulate stress on the wrist/arm and so less click allows for longer sessions and more question answered.
One place I noticed could receive improvements is the vocabs multiple-choice quiz.
In this quiz you need a first click to reveal the choices, a second click to select the answer, and a third click to advance from the "answer review page" to the next question.
Taking as example the Meaning->Kana vector, when you give the answer to this quiz ideally you recalled the kana reading and possibly even the kanji before revealing the multiple choices, therefore all you need from the "answer review page" you get after giving your answer is just a quick glance to the kanjis, to make sure you recalled the correct thing.
Given that, that third click to every answer is an extra burden that could be skipped if the quiz would automatically advance after few seconds in the "answer review page".
The optimal way for this to work would be the same way as what happens on youtube when you reach the end of a video, a circular "loading timer" appears and unless you click to pause it, it advances to the next video.
So if users could also set the speed at which the questions advance from the "answer review page" to the next question, one could set it to "2.5 seconds" and either continue quizzing smoothly, or tap the spacebar in the "answer review page" which in this mode rather than advancing the quiz would instead pause the auto-advance feature at the first tap, so one could take extra time (if needed) to more carefully inspect the kanjis or example sentences, and then advance the quiz again at the second tap.
Doing some math, if a person which has a "physical limit" of 500 questions a day (1500 clicks) then would end up reducing those click to 1000 to achieve the same ammount of answer, and those 500 clicks saved could be used to reach a new peak of 750 questions a day, that's a 33% boost to all the users :D
How does this sound? :P