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 = 'ru' 
  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 (4) 
    OR storefronts_currencies.storefront_id IS NULL
  ) 
ORDER BY 
  a.position

Query time 0.00082

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "a.position",
      "temporary_table": {
        "table": {
          "table_name": "a",
          "access_type": "ALL",
          "rows": 27,
          "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": ["devmotorsbay.a.currency_id", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(b.lang_code = 'ru')"
        },
        "block-nl-join": {
          "table": {
            "table_name": "storefronts_currencies",
            "access_type": "index",
            "key": "PRIMARY",
            "key_length": "8",
            "used_key_parts": ["storefront_id", "currency_id"],
            "rows": 7,
            "filtered": 100,
            "using_index": true
          },
          "buffer_type": "flat",
          "buffer_size": "27Kb",
          "join_type": "BNL",
          "attached_condition": "trigcond(storefronts_currencies.storefront_id = 4 or storefronts_currencies.storefront_id is null) and trigcond(storefronts_currencies.currency_id = a.currency_id)"
        }
      }
    }
  }
}