If you're on Linux, you have a lot more options to affect the system. You could try running Heroic Launcher through trickle: https://unix.stackexchange.com/questions/34116/how-can-i-limit-the-bandwidth-used-by-a-process

Ideally this would be implemented on the client side, i.e. Heroic Launcher, but there seems to some challenges in making that happen: https://github.com/Heroic-Games-Launcher/HeroicGamesLauncher/issues/597