thank you, interesting link. Does TCP Fast Open have anything to do with the "slow start", which a few major websites are known to violate[1][2] ? just out of curiosity. It doesn't look so, at a first glance.
It sounds like it would allow a client to send data (HTTP request) before the server has acknowledged that the TCP connection is open.
Not really. Slow start is a precaution to avoid the network from getting congested; whereas TCP fast open is essentially trying to reduce the completion time of small TCP requests.