SELECT 
  * 
from 
  cscart_user_profiles 
WHERE 
  user_id = 0 
  AND is_default = 'Y'

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_user_profiles",
          "access_type": "ref",
          "possible_keys": ["uid_p"],
          "key": "uid_p",
          "key_length": "3",
          "used_key_parts": ["user_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "cscart_user_profiles.is_default = 'Y'"
        }
      }
    ]
  }
}