Probably for user containers, but I've only ran it as root generally to avoid those sorts of limitations so haven't noticed any issues - I rarely use docker and only use for quick testing and then switch to non-docker installs

Rootless Podman uses slirp4netns by default. The default will soon change to pasta. Pasta has better performance than slirp4netns. For best performance if your container supports it, use systemd socket activation because the traffic over the activated socket will have native network performance.