I did mean thinking speed. Note the exchange where Harry blames Quirrell for not thinking of stuff in time, and Quirrell retorts that even smart people miss stuff and need time to reach their conclusions. An author has usually much more time than the characters and the benefit of controlling the world entirely, so he can make the characters more intelligent than him in that they're able to think faster and better under pressure, and that they're able to legitimately draw the right conclusions from the evidence that's been left available to them.