If you have a lot of icons, documents, etc., on your desktop, clean it up. OS X treats each and everyone of them as an application. Put them into a folder on your home folder on your hard drive labeled as Desktop stuff and then make Aliases of the stuff you really want on your desktop. Aliases are not treated as applications. You will be surprised at how much this will speed things up.
Repair permissions. That's in Applictions/Utilities/Disk Utility and click on the Hard drive and then select Repair Permissions. It will do it automatically. It may take a few minutes up to a half an hour. . . but that can speed up things also.
Go into console also in Utilities and check the various error messages. . . see if something is wrong there. Clear the messages.
Check your Activity monitor also in Utilities and click on the CPU % and see if anything is taking an inordinate amount of your CPU attention. See what it is. . . and if you really think it is something you recognize.
bmfl. THANKS