I only just noticed this the other day when I was trying to resolve another unrelated problem. I was using Task Manager to keep an eye on my CPU usage which spiked to 100% usage every 120 seconds or so.

The default behaviour of Task Manager is to update once every second, and if you check the Update Speed option in the menu, “Normal” is already selected. That would lead you to believe that “Normal” is 1 second (High is every 0.5 seconds and Low is every 4 seconds).  The problem is, if you ever select anything in this menu – be it High, Normal, Low, or Paused – “Normal” will set the update interval to two seconds. It is impossible to get back to the default (and most useful) update interval of one second.  Simply, it’s bugged.

To get back to the default behaviour, you need to:

• Open Regedit.exe
• Browse to
  HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\TaskManager
• Delete the string Preferences
• Restart Task Manager

You will then have the default behavior of having an update interval of one second.  Thanks to Trogdor on the Firing Squad forums for finding this solution.