SELECT 
  a.*, 
  b.description 
FROM 
  cscart_currencies as a 
  LEFT JOIN cscart_currency_descriptions as b ON a.currency_id = b.currency_id 
  AND lang_code = 'en' 
  LEFT JOIN cscart_storefronts_currencies AS storefronts_currencies ON storefronts_currencies.currency_id = a.currency_id 
WHERE 
  1 
  AND a.status IN ('A', 'H') 
  AND (
    storefronts_currencies.storefront_id IN (1) 
    OR storefronts_currencies.storefront_id IS NULL
  ) 
ORDER BY 
  a.position

Query time 0.00922

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "storefronts_currencies",
          "access_type": "system",
          "possible_keys": ["PRIMARY", "idx_storefront_id"],
          "rows": 0,
          "filtered": 0,
          "const_row_not_found": true
        }
      },
      {
        "read_sorted_file": {
          "filesort": {
            "sort_key": "a.position",
            "table": {
              "table_name": "a",
              "access_type": "ALL",
              "rows": 4,
              "filtered": 100,
              "attached_condition": "a.`status` in ('A','H')"
            }
          }
        }
      },
      {
        "table": {
          "table_name": "b",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["currency_id", "lang_code"],
          "ref": ["cscart.a.currency_id", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(b.lang_code = 'en')"
        }
      }
    ]
  }
}