In my opinion, completing undergrad in 2 years could be a mistake, unless you have a plan to publish in top tier journals and or complete requisite internships etc. before you graduate
Admission to top grad programs is significantly different than to undergrad programs. For one, the faculty is usually intimately involved in the admission process. These guys don’t usually care much about your gpa and how quickly you completed your degree. They want to know who were your professors, what you did with them and what they have to say about your ability to succeed. They also want to know how you did in specific coursework that they feel is relevant to the grad program you are applying to.
That said, if you are, for example a math prodigy who has essentially completed all the math major requirements in high school at the local flagship state uni. who is attending undergrad at Harvard to essentially complete a few electives, do research and publish with some professors there, then yeah, it would be okay to knock out undergrad in 2 or 3 years and start your PhD. (Don’t recall if they let you in less than 3). Otherwise I wouldn’t recommend it.
Most kids who have 12-20 AP classes in high school usually stick around to build research and internship experience as well as explore their interest in other fields. Sometimes they may even hold off graduating by not taking some required freshman elective such as government or dance or physical ed. Then, they take the undergrad courses, research and internships they want, often exceeding the total number of courses required to graduate. The goal is to get into the best grad school