Three protocols carry the same concurrent streams across the wire. HTTP/1.1 makes each request cross one at a time; HTTP/2 lets many streams share one boat (a single TCP connection); HTTP/3 gives each stream its own boat over QUIC. Pick a protocol, tune the load, then drop a packet and watch who is bound to wait.
How many requests race at once. More streams make the shared-boat stall more dramatic.
Request size — bigger responses spend longer on the wire and more time exposed to loss.