SELECT 
  COUNT(cart_id) 
FROM 
  cscart_cp_sfl_products 
WHERE 
  session_id = 1726872720

Query time 0.00112

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_cp_sfl_products",
      "access_type": "index",
      "key": "PRIMARY",
      "key_length": "19",
      "used_key_parts": ["cart_id", "session_id", "user_id"],
      "rows": 1368,
      "filtered": 100,
      "attached_condition": "cscart_cp_sfl_products.session_id = 1726872720",
      "using_index": true
    }
  }
}