A WordPress child theme is a theme that inherits the functionality of the parent theme – https://developer.wordpress.org/themes/advanced-topics/child-themes/ . With a child theme, you can make modifications or add additional functionality to the parent theme. The child theme offers the safest and easiest way to modify an existing theme, whether you want to make a few small changes or extensive ones ( https://xstore.helpscoutdocs.com/article/32-child-theme )
If you do not plane to do additional customization in theme files, the child theme is not required.
Custom CSS code can be added in Theme Options > Theme Custom CSS.
Just installing the child theme doesn’t affect the site speed.
Read this article also – https://xstore.helpscoutdocs.com/article/100-how-to-speed-up-my-website
