I call this "Brain Idling". When we're young our brains have a lot of grey matter and we're addicted to information accumulation, talking, chatting, and being mentally stimulated. As we age we stop learning (no school, no collage, nothing new going on in life), have less grey matter and start going on auto-pilot. When we drive to the store, when we shop for groceries, at work, at home. We've memorized our lives so well we no longer think about what we do, we just naturally do it on auto-pilot and our minds start to "idle" a lot more that usual. The film in our camera-like brains is snapping images at a much slower pace. A lot of time is passing between mental snap-shots. So when we look back (playback) the events of the day (all the things we remembered) it feels like time has flown by at light speed.
I freaked out over how fast time has flown by after I hit 21 and tried out an experiment. I installed "talking clock" for windows on my pc and made it announce the time every 5 minutes. Yes. For the whole day. Every day. After the 2nd day of using this technique time slowed down by massive amounts. A day felt like a week. A week felt like a month. It was insane. I felt like I was young again and the world was moving at a glacial pace. There are downsides of course, it's unsustainable. You have to take breaks every other day or so otherwise you get used to hearing the time and ignore it. It can also get exhausting when your brain can't take a break and daydream or idle and think about nothing. But it does work. I wanted to write a nice big article on it but I'm so swamped by work on projects and new clients needing UI/UX consulting that I just keep putting it off. On the plus side I've discovered a way to slow down time (at least for me).