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 0.00016

JSON explain

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