Six days to create the whole universe is actually very impressive if you ask me.
Why six days specifically? The Bible often correlates the six days of creation and seventh day of rest with our work week. God often does things with several goals in mind. We get the idea of a week from the creation week. God likes to do things in an orderly fashion. Before there was time, He decided He will divide time into weeks. And that it would be good if the creatures he creates can do something for six days and then rest and recharge for a day. And then He thought it would be good to do an example of what a work week should look like by doing His work in six days and then resting.