Because slow programmers don't program first. They program last, and do "big design up front" on paper. This is because the cost of a bad idea is hours wasted, and despite the logical coherence of various "expected value" arguments, students are loathe to risk having wasted multiple hours doing the wrong thing. Because humans' internal risk functions and utility evaluation functions are not linear and bayesian, people who code slowly also code differently, and those differences further slow them down with the goal of having less wasted time. In an effort to reduce their variance, they unknowingly increase their time-to-solution.