Some "in-place" upgrades from Windows Vista to Windows 7 may take users over 20 hours to complete, according to Microsoft.
The best that users can hope for is a 1 hour and 24 minute process, said Chris Hernandez, who works in the Windows deployment team, in a company blog published last week.
So-called "clean" installs, where the user overwrites an existing edition of Windows to end up with the OS, but no former data or applications, take less time: from 27 to 46 minutes.
Hernandez said the in-place upgrade times were obtained from lab machines in three different configurations - labeled low, mid-range and high-end - with three simulated users: a medium user, a heavy user and a super user. The profiles differed in the amount of data and the number of applications that were on the PC before the upgrade to Windows 7.
The medium user profile, for example, assumed 70GB of data and 20 applications; the super user profile, on the other hand, contained 650GB of data and 40 applications.
"One of the main goals with Windows 7 in general has been to be better than Vista," explained Hernandez on the blog.
"As part of the Windows Upgrade team we have tracked Windows 7 upgrade performance using Vista as our baseline comparison."
Microsoft's goal, he added, was to make an in-place upgrade from Vista Service Pack 1 (SP1) to Windows 7 at least 5% faster than an in-place upgrade from Vista SP1 to a new copy of Vista SP1.
Hernandez claimed Microsoft's testing showed, "that Windows 7 upgrade time is faster or equal within a 5 percent threshold to the Vista SP1 upgrade time." A table published in his blog post showed that in every situation, a Windows 7 upgrade was more than 5 percent faster than one using Vista.
But the data also illustrated that many of the in-place upgrade scenarios took an extremely long time. Of the 16 scenarios - three each for medium and heavy profiles, two for the super profile, with the tests run for both the 32- and 64-bit versions of Windows 7 - only four clocked in at less than two hours, and only eight in under three hours.
The speed record, according to Microsoft's testing, was the medium user profile upgrading to Windows 7 64-bit on a high-end PC, at just under 84 minutes.
But most of the in-place upgrades couldn't come close to that mark. The slowest 32-bit upgrade, for instance, was a super user on a medium machine - Microsoft didn't bother testing that profile on a low-end system - that crossed the tape at an amazing 20 hours and 15 minutes. The longest 64-bit upgrade was 10 hours, 8 minutes.
Those times sparked Hernandez to defend the time trials, which some reports had categorized as taking almost a full day.
"The 'super user' profile is not a normal user; rather, it's the user profile that represents the extreme power-user who's working with an enormous data set and a large number of installed applications," said Hernandez.
"This user profile is not representative of what most 'regular' users, who typically have a much smaller data set and would therefore experience a much, much shorter upgrade time."
Even so, Microsoft's data showed that so-called "medium" users, those with 70GB of data and 20 applications - would spend between 1 hour and 40 minutes and 2 hours 50 minutes doing a 32-bit upgrade. (The more powerful the PC, the faster the upgrade, according to Microsoft.)
Heavy users, which Microsoft posed as people with 125GB of data and 40 applications, would need between 2 hours and 40 minutes and 5 hours and 43 minutes to do the same upgrade.
Clean installs, on the other hand, were much faster, not surprisingly since no data or applications were retained. A clean 32-bit upgrade took between 27 and 39 minutes, while a clean 64-bit upgrade clocked in at between 30 and 47 minutes.
Those marks, of course, do not account for the time a user would spend restoring previously-backed up data and various settings, and re-installing applications.
Microsoft did not test Windows XP-to-Windows 7 upgrades, even though a clean install is the only upgrade path between those two operating systems. But users who start with XP and migrate to Windows 7 should expect times similar to the "clean" scenario Microsoft benchmarked.
Again, those times will not include restore data and settings, or re-installing applications; the latter can be a laborious process, with some major programs, such as Microsoft Office, taking as nearly as long as the operating system to re-install.
For clean upgrades, users - those beginning with either Windows XP or Vista - can use the Windows Easy Transfer utility that comes on the Windows 7 DVD to help them back up and restore settings and data.
More information about the PC configurations and user profiles that Microsoft tested, and the time trial results, can be found on Hernandez's blog.