How does Norton insight work? Scan time does not seem to be reduced

I have NAV 2009 on XP SP3. After clicking on Norton isight it shows only 5% of files need scanning.

 

Yet if I do a manual scan of my C:\ drive that contains the OS and program files (all my documents are on a second physical hard drive, D:\) It still takes about as long (~90 minutes) as it did when I did my first manual scan (months ago).

 

Scan performance profile is set to high trust.

 

I had expected that it would now take only about 5% as long, i.e. about 5 minutes.  Am I doing something wrong, or have I just misunderstood?wrong, or