I’ve devoted several columns to hardware solutions - replacing old hard drives with fast new SSDs, adding more RAM, and increasing storage capacity using an external drive - but there are software solutions, too.
“My Mac used to be fast, but now it’s running so slow.” I’ve heard many versions of this complaint, and they’re always factually true, not just opinions: Macs do become sluggish over time, even if all of their chips and hard drives are working like new.