"handshakes and rate control start" This is a common way to describe congestion control which is a bit simple and reductive. Given 2 endpoints on the network,

"handshakes and rate control start" This is a common way to describe congestion control which is a bit simple and reductive. Given 2 endpoints on the network, you want to compute a congestion window for the entire flow rather than per-stream. Because 1) You can more accurately compute a single congestion window given aggregate stats from N streams rather than computing N congestion windows given stats from 1 stream each. 2) The congestion window is a shared resource across streams, which permits better network utilization The other benefit of QUIC, coming from multiplexing streams over UDP rather than TCP, is that the "blast radius" arising from the consequences of a lost packet is better contained. It only impacts the stream that contained the packet; the rest of the flow can make progress uninhibited. So head-of-line blocking is not as severe.