If it needs to be real time enough for trading then surely even the 15 ms for a non-TLS connection is too much? HFT system latency is measured in nanoseconds these days and I'm pretty sure the HTTP stack has too much overhead to be used in that domain.
Don't think a real time trading website has anything to do with HFT. Still, some of these trading websites that target day traders for customers need to be pretty fast.