SELECT 
  vvr.*, 
  pfd.description, 
  pfvd.variant 
FROM 
  cscart_vd_vendors_relationships AS vvr 
  LEFT JOIN cscart_product_features_descriptions AS pfd ON pfd.feature_id = vvr.feature_id 
  AND pfd.lang_code = 'es' 
  LEFT JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfvd.variant_id = vvr.variant_id 
  AND pfd.lang_code = 'es' 
WHERE 
  brand_id = 6

Query time 0.00075

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "vvr",
          "access_type": "ref",
          "possible_keys": ["brand_id"],
          "key": "brand_id",
          "key_length": "4",
          "used_key_parts": ["brand_id"],
          "ref": ["const"],
          "rows": 18,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "pfd",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["feature_id", "lang_code"],
          "ref": ["live_clonned.vvr.feature_id", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(pfd.feature_id = vvr.feature_id and pfd.lang_code = 'es')"
        }
      },
      {
        "table": {
          "table_name": "pfvd",
          "access_type": "ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "ref": ["live_clonned.vvr.variant_id"],
          "rows": 12,
          "filtered": 100,
          "attached_condition": "trigcond(pfvd.variant_id = vvr.variant_id and trigcond(pfd.lang_code = 'es'))"
        }
      }
    ]
  }
}