On 27th of February, Alvaro Herrera committed patch:
Allow BASE_BACKUP to be throttled A new MAX_RATE option allows imposing a limit to the network transfer rate from the server side. This is useful to limit the stress that taking a base backup has on the server. pg_basebackup is now able to specify a value to the server, too. Author: Antonin Houska Patch reviewed by Stefan Radomski, Andres Freund, Zoltán Böszörményi, Fujii Masao, and Álvaro Herrera.