Overlapped Culling and Occlusion Culling

You can access both the Overlapped and Occlusion Culling by going to Settings (F6) and selecting the Renderer option on the left and go to the Performance tab. Here you will be able to enable/disable either or both.

Occlusion culling will prevent objects that are fully obscured by another object from drawing, this means that only what is necessary is being drawn in each frame, this will help reduce the cull time if you're seeing a high number there in the diagnostics.

Overlapped culling will be useful if the Diagnostics Window is showing a high cull number in the cull/draw, this will offload the cull to another thread allowing it to prepare the next frame for drawing whilst the current frame is still drawing, the drawback to this is that it adds a frame of latency, meaning that this may not be ideal for using immersive modes.