Occlusion culling usually helps the most in dense city scenes or indoor scenes with a lot of rooms. It’s usually not very helpful in scenes where all geometry is visible and in the open.
The occlusion culling process occurs off of the main render thread. It is designed to not stall the main render thread unless it saving more work than it is costing.
That said - we are still working on ways to save on CPU side processing for the occlusion culling threads.