Boosting Mac’s performance

A friend tells me he knows how to make my mac faster. That is exactly what I need. My old Mac still works, but barely! It needs almost five minutes to completely go through the boot process. Of course, opening apps is a challenge. It takes my Mac twice as long to open the same app compared to when it was new. I can't really afford a new laptop, so I must boost the speed of the one I have. The YouTube video that my friend suggested looks promising. It is perfect for my needs. If it works, it will restore my Mac to its original speed.