There is a program called TuneUp Utilities that i like. seems not to do any harm. it is amazing how many "left ins" it finds the first time it runs. it is also very good at getting rid of the crap that decrapify does not dig out.

i think hard drives start slowing down by the time they are 50% full. stick another hd in your machine and store all the pics, videos and music on it. hds are very cheap, and very easy to install in today's world.