Does is_admin() really provide a plugin performance improvement?

The is_admin() function is basically used to check whether the running request is routing on /wp-admin/. Calling the is_admin() conditional function, you can restrict files you only want to load when a request goes through the /wp-admin/.

Note that is_admin() and is_super_admin() are not the same and both handle different conditions.

Looking forward, when sending an ajax request, it goes through the /wp-admin/ folder, so it would be nice to do:

if ( is_admin() || wp_doing_ajax() ) {
    // code here
}

Talking about best usage, I would say optimization, efficiency, and speed. Loading the entire class files at once by doing if ( is_admin() ) { ... } is never a good practice. You should also extend the check to an independent action that requires a condition. For example:

if ( is_admin() ) {
     require_once '/path/to/file/';
    if ( $_GET['type'] ?? false ) {
        include_once '/path/to/another/file/';
    }
}