The OP might find that your hard drive is getting full. When they do, computers slow right down. They use the hard drive for normal operation, not just saving stuff. They are always shunting stuff in and out of the hard drive - this might be the 'paging' that Saab talks of ? I don't know exactly what PCs do in the background, or why they do it.
If the hard drive is nearly full though, it really slows down the computer.
I like the sound of Ccleaner, if it works on Apple OS.
My Mac was getting slow and cranky and my son discovered that the disc was nearly full. This could not be true, since we had not put that much in, but we found that I had clicked the wrong button when taking some screen shots, and instead of a still screen shot, I had recorded 30 hours of screen activity ! And I did this four times !!
He cleared all that out and we are back to normal.