“Creation ex nihilo” means creation from nothing, which signifies that creation means bringing into being out of non-existence. Creation from nothing is beyond what humans can do no matter what degree of talent they possess.
But the usage of Hebrew (bara or asah) and English (create or make) include both creation from nothing and creation from something. There must be two different kinds of creation acts. Making a man out of dust is an act of creation, too. This means the difference between humans creating things and God creating things in the second sense is a matter of degree.
Since creation in the second sense takes time for us, it makes sense that it would take time for God, too, though less time that it would take us. It takes us a long time to move a mountain; less so for God.