In the two cases mentioned, there isn't any extra _response_ time.

> servers cannot periodically advertise their load measures to clients

This is an asynchronous mechanism - you can use polling, push, or queue, or whatever. Individual responses don't pay the cost.

> staple current load reports to query responses, which the client can cache and use for a period of time

This is just a few extra bytes in the response, which likely has negligible cost. One can fairly reasonably say that responses don't pay any cost.

Obviously there are other costs - it just depends on what you're trading off between on whether it is the right decision.