Just to clear up some confusion regarding this setting.
1. Umbra culling affects how objects that are not visible to you are culled out before they are sent to the GPU for rendering. This is a draw call optimization, but just figuring out whether something should be rendered or not is not a cheap calculation, and can at times prove more expensive than just going ahead and drawing the object itself. This is why you were seeing the performance hit.
2. The setting the game used before the last update was performance intensive. You no longer need to type 'umbra culling' on the console anymore. In the video settings menu, there is a occlusion quality setting. The game will default it to Normal, and you should not put it to high unless you are ok with the performance hit.
3. The difference between normal and high is basically - in normal if you get to very low frame rates you might see stuff pop in if you rotate your camera very quickly. High will ensure that this never happens but at a consistently higher runtime cost as you've noticed. Otherwise, there is no difference between them, and I'd recommend you keep the default setting which is normal.