I think you are right. Someone who understands the replication code very well advised us to use that log message as a way to measure how much time it takes to send all the missing WAL to a remote standby on a slow WAN link. While it worked well for all measurements, when we use a middleware which caches a lot of traffic on the sender side, this log message was very counter intuitive. It took several more minutes for the standby to actually receive all the WAL files and catch up after the message was displayed on the master side. But then as you said, may be relying on the message was not the best way to measure the time.