Knowledge of delayed ack and nagle's algorithm wasn't necessary to solve.
The explanation didn't mention the flushHeaders call, which is apparently the fix. I didn't run any tests, just looked at the JS and figured that sending 2 packets is worse than sending 1 w.r.t. latency.
It's also a pretty strong intuition that the client side tends to have issues, since the server is usually well-tested and standardized. Also, very often people are measuring wrong, so checking the JS to be sure the time recorded is accurate is also important.