[toast0]

TLS certs won't count against your initial congestion window, although there's still a bit of overhead.

If there's a session/ticket resumption, the server won't send a certificate, but it does still need to send a negotiation finished message, and it may likely want to send new tickets (I'm not sure if you can delay that though). In TLS 1.3, the client may send the request as early data, if not the request will come as the beginning of the second round trip, so the congestion window will have opened more for the response.

If it's a full handshake, the certificate is part of the first round trip, and the content is in the second round trip; the cert won't count against the congestion window, because it must have been received before the client sent the http request.