What do commercial 'performance cleanup tools' really do?

My PC has been running slower over time, and I'd like to take non-drastic
action to restore the speed as much as I can. I've already run a registry
cleaner, defragmented the disk, and uninstalled a few programs that I
never/rarely used. So what else is there? What else can I do?
I see these ads for products that improve your PC performance, but I want to
know *what* they are doing to my computer. (I'm reluctant to blindly turn my
computer inner workings over to *anything*, let alone something making wild
"Trust us!" claims!) Does anyone know what steps the commercial products take
to speed up a computer? Do you have suggestions for suitable individual
products I can use to do it myself?
Thanks in advance for any replies!
-Paul S