A slow PG query

Sample.sql

SELECT 
  u.id, u.name_fk, rel.nub_fk, u.dataset_key, u.constituent_key,
  CASE WHEN u.is_synonym THEN par.parent_fk ELSE u.parent_fk END as parentKey,
  CASE WHEN u.is_synonym THEN coalesce(parn2.canonical_name, parn2.scientific_name) ELSE coalesce(parn.canonical_name, parn.scientific_name) END as parent,
  CASE WHEN u.is_synonym THEN u.parent_fk ELSE null END as acceptedKey,
  CASE WHEN u.is_synonym THEN parn.scientific_name ELSE null END as accepted,
  u.basionym_fk, basn.scientific_name as basionym,u.is_synonym, u.pp_synonym_fk, 
  u.origin,u.status,u.nom_status,u.rank,u.references,n.scientific_name, n.canonical_name, 
  n.type as nameType,
  authorship_full(n.authorship,n.year,n.bracket_authorship, n.bracket_year) as authorship,
  pub.citation as publishedIn, accto.citation as accordingTo,u.taxon_id, 
  u.source_taxon_key, u."references",ku.id as kid, 
  coalesce(kn.canonical_name,kn.scientific_name) as kingdom,pu.id as pid, 
  coalesce(pn.canonical_name,pn.scientific_name) as phylum,cu.id as cid, 
  coalesce(cn.canonical_name,cn.scientific_name) as class,ou.id as oid, 
  coalesce(onn.canonical_name,onn.scientific_name) as order,fu.id as fid,
  coalesce(fn.canonical_name,fn.scientific_name) as family,gu.id as gid,
  coalesce(gn.canonical_name,gn.scientific_name) as genus,sgu.id as sgid,
  coalesce(sgn.canonical_name,sgn.scientific_name) as subgenus,su.id as sid,
  coalesce(sn.canonical_name,sn.scientific_name) as species,u.num_descendants,
  u.modified, u.last_interpreted, m.downloaded as last_crawled, u.deleted,
  u.remarks, u.issues 
FROM 
  name_usage u 
  JOIN name n ON u.name_fk=n.id 
  LEFT JOIN dataset_metrics m ON m.dataset_key=u.dataset_key AND m.latest 
  LEFT JOIN nub_rel rel ON u.id=rel.usage_fk 
  LEFT JOIN citation pub on u.name_published_in_fk=pub.id 
  LEFT JOIN citation accto on u.according_to_fk=accto.id 
  LEFT JOIN name_usage ku on u.kingdom_fk=ku.id 
  LEFT JOIN name kn on ku.name_fk=kn.id 
  LEFT JOIN name_usage pu on u.phylum_fk=pu.id 
  LEFT JOIN name pn on pu.name_fk=pn.id 
  LEFT JOIN name_usage cu on u.class_fk=cu.id 
  LEFT JOIN name cn on cu.name_fk=cn.id 
  LEFT JOIN name_usage ou on u.order_fk=ou.id 
  LEFT JOIN name onn on ou.name_fk=onn.id 
  LEFT JOIN name_usage fu on u.family_fk=fu.id 
  LEFT JOIN name fn on fu.name_fk=fn.id 
  LEFT JOIN name_usage gu on u.genus_fk=gu.id 
  LEFT JOIN name gn on gu.name_fk=gn.id 
  LEFT JOIN name_usage sgu on u.subgenus_fk=sgu.id 
  LEFT JOIN name sgn on sgu.name_fk=sgn.id 
  LEFT JOIN name_usage su on u.species_fk=su.id 
  LEFT JOIN name sn on su.name_fk=sn.id 
  LEFT JOIN name_usage par on u.parent_fk=par.id 
  LEFT JOIN name parn on par.name_fk=parn.id 
  LEFT JOIN name_usage par2 on par.parent_fk=par2.id 
  LEFT JOIN name parn2 on par2.name_fk=parn2.id 
  LEFT JOIN name_usage bas on u.basionym_fk=bas.id 
  LEFT JOIN name basn on bas.name_fk=basn.id 
WHERE 
  rel.nub_fk=7011685 
ORDER BY u.dataset_key, u.id 
LIMIT 10 OFFSET 0;