Weirdly enough I think QUIC often does something that is not much better because of address validation. It is not safe for the QUIC server to send a bunch of large packets back to the client source address without being confident the client controls the address. We run QUIC in production so I’ll grab an example conversation to show how many round trips connection setup takes. I guess we might not be optimising connection setup correctly because we are using long running QUIC connections to transport our own protocol and I don’t care how long the connection setup takes.

so we are using a slightly older version of QUIC than the RFC and using go-quic on server and client. and this is what i see when creating a connection:

    Client: Initial (1284 bytes) includes client hello
    Server: Retry (166 bytes) includes retry token
    Client: Initial (1284 bytes) including retry token and client hello
    Server: Server Hello/Encrypted Extensions/Cert 
    Request/Certificate/Certificate Verify/Finished (1284 bytes) 
    Client: Certificate/Certificate Verify/Finished (1284 bytes)

address validation is covered in RFC 9000:

https://datatracker.ietf.org/doc/html/rfc9000#section-8.1

probably what go-quic does is not optimal because you don't always have to validate the address.

   Prior to validating the client address, servers MUST NOT send more
   than three times as many bytes as the number of bytes they have
   received.  This limits the magnitude of any amplification attack that
   can be mounted using spoofed source addresses.  For the purposes of
   avoiding amplification prior to address validation, servers MUST
   count all of the payload bytes received in datagrams that are
   uniquely attributed to a single connection.  This includes datagrams
   that contain packets that are successfully processed and datagrams
   that contain packets that are all discarded.

...

    A server might wish to validate the client address before starting
   the cryptographic handshake.  QUIC uses a token in the Initial packet
   to provide address validation prior to completing the handshake.
   This token is delivered to the client during connection establishment
   with a Retry packet (see Section 8.1.2) or in a previous connection
   using the NEW_TOKEN frame (see Section 8.1.3).