As a matter of separation of concerns bandwidth limits really belongs in the OS, which is responsible for the allocation of access to resources by individual processes, and for enforcing any relevant policy thereof. Fortunately this is reasonably easy to tickle at that level. Either
$ ionice -c 3 github-backup or the somewhat more cumbersome, albeit more flexible and robust, $ systemd-run --pty --uid="$(id --user)" -p "IOWriteBandwidthMax=/dev/sda 0.5M" -- sh -c "cd $(pwd) ; github-backup"
