I got around this problem by temporary limiting the server’s bandwidth using wondershaper. This is just a shell script which simplifies handling of iproute’s tc command.
Example at Ubuntu 18.04: limit bandwidth to ancient 2048 kBit download and 512 kBit upload like this:
wondershaper -a eth0 -d 2048 -u 512
Don’t forget to reset it after successfully bootstrapping Discourse:
wondershaper -c -a eth0
You just need the shell script wondershaper by the way. No need to install any service.