How does the creation of an AMI on AWS work and why is the creation time unpredictable?

Creation date: September 23, 2019 at 9:22:06 PM UTC+2

21:48: still waiting for completion

21:53: completed

Sometimes it takes 15 minutes and sometimes longer. How does a creation of an AMI work and why is the creation time unpredictable?

Answer

Are you creating the AMI from the same type of EC2 instance? If so what type of instance is it (e.g. t2, m4 large, etc)?