SELECT 
  COUNT(
    DISTINCT storefronts.storefront_id
  ) 
FROM 
  ushop_storefronts AS storefronts 
  LEFT JOIN ushop_storefronts_currencies AS currencies ON storefronts.storefront_id = currencies.storefront_id 
WHERE 
  1 = 1 
  AND (
    currencies.currency_id IN ('4')
  )

Query time 0.00071

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}

Result

COUNT(DISTINCT storefronts.storefront_id)
0