They don’t quite do the same things. Systemd will do stuff like ensure the service is restarted if it ever crashes. It can also make sure system-level dependencies are up and running (“service B depends on service A, so wait for A to be up before trying to start B”).
Performance is not an issue in most docker setups you would ever use, correct.
True, Docker-compose has a lot more overlap with systemd.
But it doesn’t have system-level dependencies. For example, in systemd I can wait for a network interface to be up and have an IP assigned by DHCP. As far as I am aware, docker compose knows about the docker network and its own containers, but not the system more broadly.
Also, you will likely want it to run for a long time, so something has to trigger the docker-compose process to start and restart it. You might want it to restart in case the OOM killer knocks it over. That daemom stuff is what systemd is good for.
Performance is not an issue in most docker setups you would ever use, correct.