Graduation rates are mostly correlated with admission selectivity. Colleges with stronger students tend to have higher 4, 5, and 6 year graduation rates.
This suggests that a stronger student is more likely to graduate in 4 years (or fewer), regardless of college, assuming affordability (money issues are the most common reason for not graduating at all, and may be another reason for late graduation if the student needs to reduce course loads to accommodate more work for pay). Note also that the stronger students are more likely to come into college with advanced placement, while the not-as-strong students may need remedial course work in college.
I.e. your 4.0 HS GPA student going to a college that is comfortably affordable is more likely to graduate in 4 years than your 3.0 HS GPA student going to a college that requires 20 hours of paid work per week to barely have the money to pay for it.
Going in undecided and not choosing first year courses to make progress in all of the possible majors can be another reason for graduating late. So can changing majors at a relatively late stage. Some majors with high volumes of requirements and/or long sequences of prerequisites need more careful attention to scheduling – not taking a course in a particular semester because of a desire to avoid an 8am time could mean graduating a semester or even a year late.