One of the reasons you have a slow computer is storage of unnecessary files. In this day in age storage is becoming cheap, hence we download/copy things and then forget about it. All this adds up and finding this files manually is a nightmare in itself.
If you need to know what files are taking up what kind of space Disk Inventory X for MacOS will give you a big easy to read graphic that also helpfully takes you right to the storage chugging files you want to delete. A nice alternative for Windows users is Windows Directory Statistics.
Check them out both are free.