QUIC: disabled non-immediate ACKs for Initial and Handshake. As per quic-transport-33: An endpoint MUST acknowledge all ack-eliciting Initial and Handshake packets immediately If a packet carrying Initial or Handshake ACK was lost, a non-immediate ACK should not be sent later. Instead, client is expected to send a new packet to acknowledge. Sending non-immediate ACKs for Initial packets can cause the client to generate an inflated RTT sample.