The first answer is that parallelism without priority (which is what parallel h1 is) can lead to some really horrible outcomes. Critical pieces of the page get

The first answer is that parallelism without priority (which is what parallel h1 is) can lead to some really horrible outcomes. Critical pieces of the page get totally shoved out of the way while bulky, but less important parts, get the bandwidth they need.That's why H2 and SPDY are both mux'd and prioritized. also you can definitely over-shard with h1.. as said downthread that can cause congestion problems and indeed even packet loss. For a little while pinterest had gigantic packet loss problems that were due to over sharding of images. The really annoying thing is that the "right amount of sharding" has to do with available bandwidth, the size of the resources being sent, and the latency between client and server.. Those things aren't really knowable on a generic per-origin basis when setting your links up - so the spdy/h2 approach works better in practice. if I had a criticism here, its that implementing priority right is a lot trickier than just have a bunch of independent connections. We will probably see some bad implementations in the early days until folks internalize how important it is.