For a lot of skills, you get faster as you level up. What used to take you six hours takes you three. What took you an hour takes a couple of minutes. If you are charging an hourly rate, this can be a problem. And even if you don’t, it’s easy to try to measure the worth of your work by how much time it takes to make it.
The challenge in freelancing is to recognize that this doesn’t mean your work is worth less. In fact, your work is more valuable, because in addition to leveling up on speed, you’ve probably also leveled up on quality, precision, accuracy, and any number of other measures of your skill.