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 0.00078

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.31"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "destination_links",
          "access_type": "range",
          "possible_keys": [
            "idx_store_location_destination"
          ],
          "key": "idx_store_location_destination",
          "used_key_parts": [
            "store_location_id"
          ],
          "key_length": "3",
          "rows_examined_per_scan": 11,
          "rows_produced_per_join": 11,
          "filtered": "100.00",
          "index_condition": "(`dev_yourmart_co_bw`.`destination_links`.`store_location_id` in (6,7))",
          "cost_info": {
            "read_cost": "4.36",
            "eval_cost": "1.10",
            "prefix_cost": "5.46",
            "data_read_per_join": "176"
          },
          "used_columns": [
            "link_id",
            "store_location_id",
            "destination_id",
            "warn_about_delay",
            "position"
          ]
        }
      },
      {
        "table": {
          "table_name": "shipping_delays",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "store_location_id",
            "destination_id",
            "lang_code"
          ],
          "key_length": "12",
          "ref": [
            "dev_yourmart_co_bw.destination_links.store_location_id",
            "dev_yourmart_co_bw.destination_links.destination_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 11,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "2.75",
            "eval_cost": "1.10",
            "prefix_cost": "9.31",
            "data_read_per_join": "8K"
          },
          "used_columns": [
            "store_location_id",
            "destination_id",
            "shipping_delay",
            "lang_code"
          ]
        }
      }
    ]
  }
}