Comment 4 for bug 1729192

Hmm,
that seems odd - but it doesn't have enough to recreate and debug.
As irqbalance isn't strictly required you could consider uninstalling or masking it in systemd.
But that is only a workaround.

For the sake of going forward I merged the latest irqbalance which should show up soon in 18.04.
Could you try if you can reproduce so in e.g. a KVM guest.

If so you could redirect the console and maybe even pipe it through something adding timestamps.
Sorry I'm not the biggest expert on systemd debugging - maybe there are much better options out there I don't know for shutdown delay analysis.