clearing takes forever

I am using free CCleaner v4.16.4763, home premium vista. Until recently clearing files of say 1gb with a 7 pass overwrite would happen in a few minutes, now it can be 30 to 40 minutes, sometimes longer. This is the case even when there are no other applications running. My local IT shop specialists have advised that I should not run cleaner frequently, which has been several times a week for the past couple of years +. They suggest every 2-3 months would be more appropriate. My pc is a 3 year old hp pavillion with loads of space, 3gb memory, AMD 9600 quad core processor 2.3 ghz