I reimplemented the speedtest.net backend for fun a number of years ago, right around when it was transitioning from Flash to HTML5 and offered both. I used a

I reimplemented the speedtest.net backend for fun a number of years ago, right around when it was transitioning from Flash to HTML5 and offered both. I used a PHP process running in the background using stream_socket_server(). IIRC my computer had a 5Gbps connection to localhost using HTML5, and fractionally less using Flash :> ...which meant that PHP was writing to the web browser at 5Gbps.