finder is slow, disk access is slow, 1.5 million files on disk Folders take seconds to respond; Activity monitor shows nothing; there are 1.5 million files on the computer. Is the Finder overloaded?
How much space is left on your harddrive?
Alway try te keep 15% free space. If for example you have 500GB, and you already use 470GB, this will usually seriously slow down your mac, there should at least be 75GB of free space.

More info: http://macs.about.com/od/faq1/f/How-Much-Free-Drive-Space-Do-I-Need.htm