When pg_basebackup starts, it tries to finish its work as quickly as possible. If we have a good network connection, pg_basebackup is definitely able to fetch hundreds of megabytes a second from the remote server. If our server has a weak I/O system, it could mean that pg_basebackup could suck up all the resources easily, and end users might experience bad performance because their I/O requests are simply too slow.

To control the maximum transfer rate, pg_basebackup offers the following:

-r, --max-rate=RATE 
    maximum transfer rate to transfer data directory 
    (in kB/s, or use suffix "k" or "M")

When we create a base backup, we need to make sure that the disk system on the master can actually stand the load. Adjusting our transfer rate can, therefore, make a lot of sense.