You caught my attention in your Revelation study when you asked why God took “so long” to create the world, especially your comment, “The God I worship could have created everything in an instant." I like that, but now I gotta know why He took so long!!
I chuckled when I read your comment regarding my Genesis study. I am pleased to hear my comment piqued your interest in that study, and you can hear the entire teaching in our Genesis Bible Study.
Still, I can try to give you something of an answer now (at least briefly) by covering two points. First, God chose to complete creation in seven days (rather than in an instant), in part, as a means of pointing to Christ. This idea is explained in greater detail in Hebrews 4 and elsewhere in scripture, but essentially the seventh day of creation was used by God as a picture of Christ. God the Father gave us the Sabbath as a illustration of the relationship between grace and works, between our experience of trying hopelessly to "work" our own salvation (with no opportunity for rest from that effort) verses God doing the work on our behalf while we accept His work and rest in it (i.e., resting in Christ’s work on the cross).
Hebrews 4 calls Christ our Sabbath rest (and Jesus says in Luke 6 that He is Lord of the Sabbath). He did the work of our salvation, so now we rest in that work. The six days of creation illustrate that God did all the work and mankind simply receives the benefit of that work. Likewise, God (in Christ) did the work of salvation, and believers merely rest in that work, which is why we say every day is a Sabbath for Christians.
Secondly, God created the world and everything in it in six days rather than instantly so that His purpose in His creation could be understood by how it was formed. The world was created for man, and therefore all that was done was done in such a way that man could understand he was the intended beneficiary of God's creation and not merely a player within it.
For example, when God made man on Day 6, the stage had already been set for man's entrance, including trees created already bearing fruit, a garden already prepared for man’s home, etc. Also, creating the world in six successive days gave God the opportunity to pause and declare the creation to be good at various points along the way (e.g., Gen 1:31, et al.), which is how we know that the world was made perfectly, not becoming corrupt until man introduced sin and death.