"millisecond or two" won't cut it as the only usable case would happen when there is non-trivial delay between 103 and 200 statuses.
Also I see this as more harmful than good as it'd require yet another packet sent and ack'd.
I also fail to see the usefulness. A server can already today send only the html head, flush the stream then pause before sending the body?
Most modern SPAs also have a static index.html which shouldn't take long to serve anyway.
This is going to be abused by cargo cult brogrammers who heard that "103 will make your site faster!!!1" and put every single resource as a 103 making the time to 200 even longer than the time you actually potentily gain.
