It has been said that “we do a head job on ourselves about how long something will take us to do."
Over estimating the amount of time to do something simple.
I am not sure that all people do that. Some people have a good grip on the passage of time. 
But I have ADHD-adult. I lose track of time, I am easily distracted, so simple things do take a while.  
One way that I have found to stay on track is to make a list, preferably in an order, of what I need to do in a particular area, or about a particular project. Make a plan, and then work the plan.  
Also, I find myself being rather sharper, in the morning hours, rather than in the afternoon.