SELECT 
  COUNT(
    DISTINCT storefronts.storefront_id
  ) 
FROM 
  cscart_storefronts AS storefronts 
WHERE 
  1 = 1

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "storefronts",
      "access_type": "system",
      "rows": 1,
      "filtered": 100
    }
  }
}