$wp_query->found_posts can be used to get the number of posts. But here are arguments you can use to optimize the query and avoid useless SQL retrievals:
cache_results' => false, // Turns off post caching
'no_found_rows' => true, // To optimize when pagination is not required
'fields' => 'ids' // To only retrieve IDs information, nothing else (avoid collecting all useless post data)
If you have more optimizing tips for simple queries, I would be happy to hear about them