SELECT 
  * 
FROM 
  cscart_sg_user_activity_log 
WHERE 
  user_id = 0 
  AND session_id = '' 
  AND lang_code = 'ja' 
ORDER BY 
  timestamp DESC 
LIMIT 
  50

Query time 0.01352

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 7.682563,
    "nested_loop": [
      {
        "read_sorted_file": {
          "filesort": {
            "sort_key": "cscart_sg_user_activity_log.`timestamp` desc",
            "table": {
              "table_name": "cscart_sg_user_activity_log",
              "access_type": "ALL",
              "loops": 1,
              "rows": 45891,
              "cost": 7.682563,
              "filtered": 100,
              "attached_condition": "cscart_sg_user_activity_log.user_id = 0 and cscart_sg_user_activity_log.session_id = '' and cscart_sg_user_activity_log.lang_code = 'ja'"
            }
          }
        }
      }
    ]
  }
}