In spite of the brain sometimes being referred to as a computer, it does not actual track time in any consistent way, which is why we perceive time so differently depending on what's happening. There's a whole lot going on in this piece, but essentially, our brains respond to things happening. So busy days at work go by super quickly because our brain keeps pushing its counter forward (and dead days take forever).
When we’re still and we’re bored, time goes very slowly because we’re not doing anything or nothing is happening. On the contrary, when a lot of events happen, each one of those activities is advancing our brains forward. And if this is how our brains objectively tell time, then the more that we do and the more that happens to us, the faster time goes.