For provisioning a template from the Library VMM uses BITS (Background Intelligent Transfer Service). BITS was designed to transfer large amounts of data without consuming a lot of network bandwidth by sending the data in small parts.
In many cases you might receive an error in the VMM log when you provision a new VM from a template. VMM uses BITS over HTTPS (port 443), in many cases port 443 is used for other services on the VMM server (such as IIS). This can cause conflicts with BITS.
Read the full article that I wrote for the Technet Wiki here: