Any codes entered in the Custom CSS option will be included in every single page on every page load, and they are never cached by the browser. Using a child theme, your CSS codes are in a style.css file, which the browser cache (i.e. load from the server only once, then it gets stored on the user’s computer and the browser just reads it from there, no need to load it from the server again).
The Custom CSS option is meant for quick and simple CSS modifications that spans only a few lines of codes. Any more than that, use Child Theme.