I don't think their 10 minute update interval is based on the performance limitations of the HTTP protocol, but used because it's simply not necessary to do it

I don't think their 10 minute update interval is based on the performance limitations of the HTTP protocol, but used because it's simply not necessary to do it more often. Maintaining an open full duplex connection for a single fetch request every ten minutes sounds like terrible overengineering that in the end is probably much worse performance wise. Both protocols use TCP as an underlying protocol. So the latency of the timedata from the final request should be pretty much the same (neglectible differences due to larger HTTP header that needs to be sent and parsed on receive). We don't really care about the additional round-trips before, because they don't affect the clockdrift. However, if you really do care about the clockdrift due to client-server latency there are protocols like NTP that try to calculate that drift and minimize it.