Hello folks

This has been tested with Windows 8.1 fresh install+updates and after upgrading that setup to Windows 10+updates (doing a fresh installation of the Nvidia Windows 10 drivers)
Gigabyte Z97X Gaming 7 with F7 BIOS

16GB Corsair 2133Mhz
i7 4770
Samsung 850 EVO SSD 250GB M.2
Corsair RM 750W Gold PSU

The problem I have is very clear with TERA (out of 10-12 DX9, DX11, and OpenGL games) but I'm not worried about TERA, I'm worried about the root cause and because I doubt this is the only game that can have this behavior

BIOS has the option to boot as "Other OS", "Windows 8" and "Windows WHQL" (this one should never be used according to Google):
- If I choose Windows 8 and no CSM, everything works great but TERA. TERA loading times are considerably longer (like 15 seconds more, yes it's on the SSD) and I experience choppiness. I play 10-30 seconds normally at 60FPS and then game freezes completely several times for 5 seconds, I keep playing and same thing after 10-30 seconds, eventually I just kill the game from task manager
- If I choose Windows 8 and CSM, which means UEFI first, otherwise legacy, TERA works fine and loads fast as well like any other game.

So I don't know if:
ASUS Strix does NOT have a fully compliant UEFI BIOS, or has no UEFI support at all
Gaming 7 motherboard and this model of videocard are not fully compatible for X reasons
It's an ASUS model/bios fault
It's a motherboard/bios fault
It's TERA's fault (although other users have no issues at all with 750,970,980... using Windows 8.1/10)