I get the same script time results with this two different query :

FIRST : 

{set $articles_count=$category.subtree[$category.subtree|count()|sub(1)].object.class_identifier|eq(article)}

SECOND : 

{set $articles_count=fetch(content, list_count, hash('parent_node_id', $category.node_id,															     'class_filter_type', 'include',																     'class_filter_array', array('article'),															     'depth_operator', '>',																	     'depth', 0 )) }