I think the rigid, repetitive structure is what makes days fly by. I remember one time, while I younger and time still felt slow, I was in a sort of youth camp and I spent two weeks in their spaces with many other students. Our days were scheduled from morning to night with various activities, and most of time was either participating in activities or taking a break inbetween them. At the end, I was surprised how fast two weeks went by.

Maybe boredom is needed for time to go slowly, and we just don't have time to be bored.