That's the new content system. I'm not sure what it is doing but I don't like it...
/puts on IT hat
It's doing a lot of disk operations converting the existing content to a new format. The fact is a standard 7200 rpm sata drive just isnt that fast and can be saturated with pretty easily. I havent't read what is so different about the new content format, but if it allows for installs to secondary drives I'm all for it.