La release v15.6...

<a href="{{ last_message.topic.last_read_post.get_absolute_url }}">

try:
            return TopicRead.objects \
                            .select_related() \
                            .filter(topic__pk=self.pk,
                                    user__pk=get_current_user().pk) \
                            .latest('post__position').post
        except TopicRead.DoesNotExist:
            return self.first_post()

SELECT forum_topicread.id, forum_topicread.topic_id, forum_topicread.post_id, forum_topicread.user_id,
       forum_topic.id, forum_topic.title, forum_topic.subtitle, forum_topic.forum_id,
       forum_topic.author_id, forum_topic.last_message_id, forum_topic.pubdate, forum_topic.is_solved,
       forum_topic.is_locked, forum_topic.is_sticky, forum_topic.key,
       forum_forum.id, forum_forum.title, forum_forum.subtitle, forum_forum.image,  
       forum_forum.category_id, forum_forum.position_in_category, forum_forum.slug,
       forum_category.id, forum_category.title, forum_category.position, forum_category.slug,
       T6.id, T6.password, T6.last_login, T6.is_superuser, T6.username, T6.first_name,
       T6.last_name, T6.email, T6.is_staff, T6.is_active, T6.date_joined, utils_comment.id,
       utils_comment.author_id, utils_comment.editor_id, utils_comment.ip_address,
       utils_comment.position, utils_comment.text, utils_comment.text_html, utils_comment.like,
       utils_comment.dislike, utils_comment.pubdate, utils_comment.update,
       utils_comment.is_visible, utils_comment.text_hidden,
       forum_post.comment_ptr_id, forum_post.topic_id, forum_post.is_useful,
       T9.id, T9.password, T9.last_login, T9.is_superuser, T9.username, T9.first_name,
       T9.last_name, T9.email, T9.is_staff, T9.is_active, T9.date_joined,
       T10.id, T10.title, T10.subtitle, T10.forum_id, T10.author_id, T10.last_message_id,
       T10.pubdate, T10.is_solved, T10.is_locked, T10.is_sticky, T10.key,
       T11.id, T11.title, T11.subtitle, T11.image, T11.category_id, T11.position_in_category,
       T11.slug,
       T12.id, T12.title, T12.position, T12.slug,
       T13.id, T13.password, T13.last_login, T13.is_superuser, T13.username, T13.first_name,
       T13.last_name, T13.email, T13.is_staff, T13.is_active, T13.date_joined,
       auth_user.id, auth_user.password, auth_user.last_login, auth_user.is_superuser,
       auth_user.username, auth_user.first_name, auth_user.last_name, auth_user.email,
       auth_user.is_staff, auth_user.is_active, auth_user.date_joined
FROM forum_topicread
INNER JOIN auth_user ON ( forum_topicread.user_id = auth_user.id )
INNER JOIN forum_topic ON ( forum_topicread.topic_id = forum_topic.id )
INNER JOIN forum_forum ON ( forum_topic.forum_id = forum_forum.id )
INNER JOIN forum_category ON ( forum_forum.category_id = forum_category.id )
INNER JOIN auth_user T6 ON ( forum_topic.author_id = T6.id )
INNER JOIN forum_post ON ( forum_topicread.post_id = forum_post.comment_ptr_id )
INNER JOIN utils_comment ON ( forum_post.comment_ptr_id = utils_comment.id )
INNER JOIN auth_user T9 ON ( utils_comment.author_id = T9.id )
INNER JOIN forum_topic T10 ON ( forum_post.topic_id = T10.id )
INNER JOIN forum_forum T11 ON ( T10.forum_id = T11.id )
INNER JOIN forum_category T12 ON ( T11.category_id = T12.id )
INNER JOIN auth_user T13 ON ( T10.author_id = T13.id )
WHERE (forum_topicread.user_id IS NULL AND forum_topicread.topic_id = %s)
ORDER BY utils_comment.position DESC LIMIT 1

SELECT forum_topic.id, forum_topic.title, forum_topic.subtitle, forum_topic.forum_id, forum_topic.author_id, forum_topic.last_message_id, forum_topic.pubdate, forum_topic.is_solved, forum_topic.is_locked, forum_topic.is_sticky, forum_topic.key FROM forum_topic WHERE forum_topic.id = %s LIMIT 21

SELECT utils_comment.id, utils_comment.author_id, utils_comment.editor_id, utils_comment.ip_address, utils_comment.position, utils_comment.text, utils_comment.text_html, utils_comment.like, utils_comment.dislike, utils_comment.pubdate, utils_comment.update, utils_comment.is_visible, utils_comment.text_hidden, forum_post.comment_ptr_id, forum_post.topic_id, forum_post.is_useful, auth_user.id, auth_user.password, auth_user.last_login, auth_user.is_superuser, auth_user.username, auth_user.first_name, auth_user.last_name, auth_user.email, auth_user.is_staff, auth_user.is_active, auth_user.date_joined 
FROM forum_post 
INNER JOIN utils_comment ON ( forum_post.comment_ptr_id = utils_comment.id )
INNER JOIN auth_user ON ( utils_comment.author_id = auth_user.id )
WHERE forum_post.topic_id = %s
ORDER BY utils_comment.position ASC LIMIT 1