[01HNNWZ0MV43FF]

Hm. Wikipedia tells me that a UDP headers has 4 parts: Source port, dest port, length, and checksum.

I suppose with a new protocol you could ditch the length and checksum, which (I don't know much about networking) might be redundant with the IP header and with TLS respectively. That would save you 4 bytes per packet. Nothing huge.

The reason it uses UDP is that the Internet is full of middleboxes built 10 years ago that won't get any firmware updates for another 10 years, and those boxes love TCP and we're lucky if they even pass UDP and ICMP packets. Anything else might just get dropped, and then you have to fall back. Is UDP underneath QUIC really hurting much?