You're going to look under CPU for CPU processes. You don't change anything there - you are only looking to see if there is something using a high percentage of your CPU's processing power, which will slow down other processes you try to do.
Since you haven't told us specifically what a "long time" is...
While it could be some process eating CPU cycles, a 5-7 year old hard drive with 20% free space that has never been defragged is going to be slow. Under normal use, every OS I have ever used becomes unbearably slow to me at 30-35% free space. I have friends that don't seem to notice the slow downs until they're hitting 5-10% free space or less.
Moving your photo library or anything else to an external and getting your free space back up to 40-50% will help. Running iDefrag afterwards will help immensely. Running iDefrag without freeing anything up would probably help immensely, but with 20% or less free space you're going to be defragging pretty often to keep that drive running it's fastest.
If you're going to be keeping and using that computer for another couple of years, for under $100, I'd suggest a new WD Black drive in the 1-2 TB range and either migrate or even easier, grab SuperDuper! or CarbonCopyCloner and clone your existing drive over to it. That will automatically give you a defragged volume with 60-80% free space on the new drive.
(And if you're not running Onyx (Automation tab - Cleaning and Maintenance) maybe once every 6 months to clear out the cruft, that will help also.)