Personal anecdote: I did relatively poorly for my Bachelor's, finishing with a GPA of about 2.7. I attended most lectures anywhere from 70 to 95% of the time. I was treating University like school; I went to the lectures just to listen. The week before the exam, I scrolled through the slides once, making sure I understand the material. However, understanding versus being able to fill in the answers to pre-selected types of assignments within barely more the time than it takes to thoroughly read all the questions are two very different and mostly independent skills.
During my Master's, I can count on one hand the number of times I went to a lecture hall. Instead I spent my time relaxing at home. Every once-in-a-while, I registered for a bunch of exams, which would then force me to go through the lecture slides at home and practice solving some old exams. I graduated the Master's with 4.0 GPA.