Vertical Sync drops FPS

[Your Computer]

Some games implement their own methods to handling screen tearing which doesn't decrease performance as much as v-sync, and so you often don't require turning on v-sync for those games. In those cases, v-sync could be considered as a fallback feature. Nevertheless, you should really note that Amnesia wasn't designed for outdoor environments, so comparing them to games that were designed for outdoor environments or where a lot of information would be displayed but is handled differently is not a proper comparison.

If you're looking for a DirectX game that often requires v-sync turned on, try one of Valve's FPS games, like Left 4 Dead.