If you’re using an unmodified theme, this is something you’ll never need to worry about. But if you’ve made one, teeny tiny, change to a theme, then you need to consider Child Themes.
If you’re running a website, you have a lot more responsibility than just yourself.
When you move from Single WordPress to MultiSite, you can leave your images where they are, or you can move them to the new structure.
Your security is your responsibility. Don’t trust your host to do everything right, and understand what you’re getting into.
More ways to use top to understand how to make your site run faster.
While I want to say you’ll never need caching, the truth is you probably will need it one day.
I’m the sort of person who codes something because I need it.