Comment # 4 on bug 902779 from Bruno Pesavento

I had even worse delay problems on a laptop with slow disk (SATA-I) due to
systemd flushing the system journal to disk just when Xorg is supposed to
start.
See if disabling journal to disk helps, writing "Storage=volatile" in
/etc/systemd/journald.conf  .
Not a bug proper, just a wrong default setting not suited to laptops.
In my case "Storage=auto" and "SystemMaxUse=160M" gave a workable journal with
little collateral damage.