There is a bug in Linux kernel or the input subsystem that affect any kernel above version 3.13.8-1.
3.13.8-1 is NOT affected.

After update to any Linux version above 3.13.8-1 (had the issue with 3.14.1-1 and 3.14.2-1), when the login process finishes, the system is stuck for approximately 5-10 seconds. No input is taken from the user (i.e. no reaction on key presses or mouse movements). After 5-10 seconds, the system becomes usable again. I've tested it with login into SLiM (can not enter username for the first 5-10s), auto login to OpenBox from SLiM (desktop is frozen for 5-10s), login to LightDM (same as SLiM, system frozen for 5-10s), therefor I can 100% rule out an issue with either the DM or WM.

I've attached the relevant errors from journalctl.

journal_1.log looks like irrelevant for the issue and might be related with Gigabyte inability to follow standards or whatever.
journal_2.log looks like relevant to the issue.
journal_3.log relevant as well, also notice that there are 4 seconds before timeout (I assume this waiting time causes the system freeze).
journal_4.log another relevant log.
journal_5.log relevant as well, also notice the wait time of 5 seconds before timeout.

Looks like these 4 logs are related to usb, each log related to another device (I indeed have 4 devices attached to usb ports: microsoft keyboard [attached to usb3 port], microsoft mouse [attached to usb3 port], logitech webcam [attached to usb3 port], dell monitor usb hub [attached to usb2 port]).

The motherboard is Gigabyte GA-H81M-HD3 with UEFI BIOS.
Kernel Stock.

After 5-10 seconds the system become usable again. No other issues are visible during the regular usage. All peripherals works ok.
Right now I've downgraded to the latest non affected version: Linux fractal 3.13.8-1-ARCH #1 SMP PREEMPT Tue Apr 1 12:19:51 CEST 2014 x86_64 GNU/Linux