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

Query time 0.00012

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "storefronts",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["storefront_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    }
  }
}