In general, the size of a site's files and the database will determine how long it will take to be migrated. However, aside from just its size, if the site has a large database with lots of database entries, this can also extend the staging creation time.
For instance, a 4.5GB eCommerce site with 10,000+ products, will take a lot longer than a 4.5GB site with less than 5 pages, but lots of large videos and pictures. The eCommerce site will do a lot of 'databases writes' which will slow down the staging creation process, while the other will just take time copying over the filesystem contents.