Why does it take us so long to do things even when we aren’t getting distracted?

I feel like a lot of the time people talk about adhders taking a long time to do tasks because they get distracted and start doing other things alongside it. But I find that even when I’m pretty locked in on a task and not letting other stuff get in the way, it still takes me ages to complete it. I was lucky to have Elvanse (Vyvanse) in my later years at school so I could focus on work very well. However, I always needed a lot more time to complete work compared to other students even though my train of thought was completely focused on the work. I find it takes me ages to get ready to go out. I start doing my makeup and suddenly half an hour has gone by. Time blindness is a huge factor in my adhd, but I’ve never understood how it works. I feel like I’m working at the pace of a non-adhd person but in actual fact, hours are flying by. Why does it take me so long to do things that non-adhders can do so much quicker?