public $data_transfer_rate = null
|float||The rate in bytes per seconds or NULL if the rate couldn't be determinated|
The data transfer rate is calulated by the data-transfer-time and the number of bytes that were received
alltogether. The server-connect-time and response-time are NOT included, so this is an indicator for the
server (or local) bandwidth.
This is an calculated value and gets more accurate with larger received documents.
It may not be avaliable for very small documents.