I think there needs to be some consistency in the amount of time per quiz for the random quizzes. This quiz is only 1:30 long, while scrambled capital and country quizzes are 2:30, and the random country to capital (and capital to country) are both 4:00. All are 20 questions. So, why would one quiz be half the time as another when they are so similar?
Probably because you could easily list out all of the last names of the US Presidents really quickly (even with a yellow box), so the time is shorter. That is, because there are less possible answers. Though I do agree that the time for this quiz is quite short. I was moving at a pretty good clip and had to rush to type in the last president, but I accidentally typed "Turman"...though it was too late to fix the spelling.
Time control is too frustrating for me. I am not an olympic-level word unscrambler, but I usually don't do this poorly. The people getting 100% on this must be skirting the quiz rules or getting very easy clues.
About 15 attempts and finally got them all in the time limit. I like that they're randomised, and even the anagram order changes so it's not the same quiz each time.