Here is a warning that also displays when you go into Appearance > Editor. This message appears regardless of using a child theme or not (at least in WP 5.0 +)
You appear to be making direct edits to your theme in the WordPress dashboard. We recommend that you don’t! Editing your theme directly could break your site and your changes may be lost in future updates. If you need to tweak more than your theme’s CSS, you might want to try making a child theme.
If you decide to go ahead with direct edits anyway, use a file manager to create a copy with a new name and hang on to the original. That way, you can re-enable a functional version if something goes wrong.
The only difference in performance, (which in cases of adding a little CSS is less than minimal) that I can think of is when using Appearance > Editor you are actually editing a real CSS file, so your styles will get added directly into that file. When you use Appearance > Customize > Additional CSS the orginal stylesheet isn't effected. It creates a style tag within the head section with your styles in it.
Since you already have a child theme, I would say that editing the CSS file directly is the way to go (Appearance > Editor). There is no need to add extra styles to the head section. Plus it will keep all of your styles in 1 spot making it easier to manage.
