Microsoft apologists are coming out of the woodwork to defend the insanely slow Vista-to-Windows 7 upgrade process, and the sheer amount of ignorance is breathtaking. Never have so many generated so much blather while contributing so little to the discussion at hand.
The Windows upgrade process is slow. It was slow moving from XP to Vista, and it's still slow moving from Vista to Windows 7. Anyone who has spent quality time with the process knows this, and most power users (including yours truly) opt for the "clean" install method out of recognition of the upgrade program's shortcomings.
However, this seemingly common knowledge hasn't stopped apologists from trying to justify the slowness. One popular excuse involves the size of the user profile. According to these ill-informed souls, the systems that take the longest to upgrade are the ones with many hundreds of megabytes of data files stored in their user profiles.
The problem with this explanation is that it confuses the various Windows "special" folders - Contacts, Documents, Music, Photos, and so on - with the combination of custom settings and account-specific registry data that makes up the user's current profile. This latter structure, which is embodied in the Ntuser.dat file, is normally quite small - on the order of several megabytes for most users.
Processing this amount of data is a trivial task; it should take no more than a few minutes on even low-powered hardware. And because the rest of the user's data (their "special" folder contents) consists of mostly discrete files and directories, there's really no reason for the upgrade process to touch them, though some Access Control List and metadata tweaking may occur.
Another popular explanation in the blogosphere revolves around the number of applications installed on the system. According to this theory, the more applications you have installed, the longer it takes for the upgrade process to sort through them. However, if my experience is any indicator, this excuse simply doesn't hold water. Even on my own heavily customised development workstation running Visual Studio 2008, SQL Server 2008, and ASP.Net 3.5/IIS 7.5, the entire contents of my Windows Config folder (where the various registry hive files are stored) comes out to roughly 140MB of file and directory data.
Again, it's a trivial amount to process - even a lowly netbook should be able to execute this workload in 30 minutes. Why it takes so long to complete in practice (my own upgrade from a late RTM-candidate build of Windows 7 to the final bits took nearly five hours on a quad-core system with a SATA RAID 0) simply defies explanation.
The bottom line is that Windows-based PCs are relatively simple creatures. I say chalk this one up to yet more sloppy programming on Microsoft's part and call it a day (or two, if you're upgrading from Vista).