SELECT 
  storefronts.* 
FROM 
  cscart_storefronts AS storefronts 
WHERE 
  1 = 1 
  AND storefronts.is_default IN ('Y') 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0110178,
    "nested_loop": [
      {
        "table": {
          "table_name": "storefronts",
          "access_type": "ALL",
          "loops": 1,
          "rows": 1,
          "cost": 0.0110178,
          "filtered": 100,
          "attached_condition": "storefronts.is_default = 'Y'"
        }
      }
    ]
  }
}