User profile is a collection of various settings and files under Windows (like My Documents, application settings and so on).
Over a period of time, the user profile in Windows grows large and can cause slowdowns while logging on or overall sluggishness. This can be experienced as delayed logons, slow system response etc.
To find out the size of user profiles in Windows 7 :
1. Go to Control Panel > System and select “Advanced system settings”. Then under the “Advance tab, click on “Settings” for “User profiles”.
2. This will show the list of user profiles present along with their size.
Knowing this can be a good starting point for troubleshooting system slowness/ long logon times  in combination with other factors.
Cheers.