It was years ago, and I didn't dig too deeply.

But my impression was, that sending one or two bytes at a time,
through a packetized transport (USB), and waiting for it to complete
before sending the next one or two bytes.. kinda made sense that
throughput would be slow.

I don't think the USB serial stack uses windowed transport
like TCP does.

EDIT: Mmm.. some memory is returning now..
I think it might have been the bluetooth
serial dongles here that were the slowest before those mods.

Cheers