SELECT 
  destination_links.*, 
  shipping_delays.* 
FROM 
  cscart_store_location_destination_links AS destination_links 
  LEFT JOIN cscart_store_location_shipping_delays AS shipping_delays ON shipping_delays.store_location_id = destination_links.store_location_id 
  AND shipping_delays.destination_id = destination_links.destination_id 
  AND shipping_delays.lang_code = 'en' 
WHERE 
  destination_links.store_location_id IN (6, 7)

Query time 0.00024

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "destination_links",
      "access_type": "range",
      "possible_keys": ["idx_store_location_destination"],
      "key": "idx_store_location_destination",
      "key_length": "3",
      "used_key_parts": ["store_location_id"],
      "rows": 11,
      "filtered": 100,
      "index_condition": "destination_links.store_location_id in (6,7)"
    },
    "table": {
      "table_name": "shipping_delays",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "12",
      "used_key_parts": ["store_location_id", "destination_id", "lang_code"],
      "ref": [
        "ymonline_livedata.destination_links.store_location_id",
        "ymonline_livedata.destination_links.destination_id",
        "const"
      ],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(shipping_delays.lang_code = 'en')"
    }
  }
}