Show only one post for each author ( Page loads too slow )

Edit: I just tested and it works perfectly. You’ll also want to remove pagination from the author template.

I haven’t tested it, but have you tried pre_get_posts?

function author_filter($query) {
if ( !is_admin() && $query->is_main_query() ) {
  if ($query->is_author()) {
    $query->set( 'posts_per_page', 1 );
   }
 }
}
add_action('pre_get_posts','author_filter');