As much as I complain about systemd being overly complex and the lock-in that follows:

    sudo systemctl edit --force user-1234.slice

    [Slice]
    MemoryMax=1G
    CPUQuota=10%

It's also something that's been available on the other UNIX-likes (FreeBSD/Solaris) since essentially forever.

And systemd does the exact same thing under the hood as running in a container would.

cgroups?