domain sharding gives you parallelism - but it totally bypasses congestion control. At certain levels this works in your favor - because congestion control tend

domain sharding gives you parallelism - but it totally bypasses congestion control. At certain levels this works in your favor - because congestion control tends to be overly conservative for HTTP - but at levels of sharding too high it is a disaster (packet loss, heavy buffering, reordering and huge delays to sort it all out.) Unfortunately the appropriate level of sharding isn't something the person doing the markup can know - because its based on the runtime path characteristics of each page viewer. Much better to multiplex in one congestion control context (like http/2 does) and then work on making congestion control work better (like the various blah over udp projects such as quic are trying to do). Plus if you do this you can add prioritization, which is a big deal.