LOD not improving performance as much as it should, see profiler screenshots

Hey guys,

I'm developing an Oculus Rift game, and their SDK uses its own Vsync which shows up in the profiler under 'other', in red. So in these profiler screenshots you want to ignore the red and look at the height of the green.

http://imgur.com/a/64cwy

I've got a big terrain with loads of props, and LOD has been set up on all of them. The props are all children of a single gameobject. When I reduce the LOD bias at runtime, the performance improves slightly, as you would expect. At LOD bias 0, everything is being culled. The issue is that when I then deactivate the props parent gameobject, which of course causes no visible change at all because all the props are culled anyway, the CPU time improves dramatically. To me this says there is still a lot of rendering being done even when they're all culled. The number of drawcalls doesn't change at all when the parent is removed.

Anyone have any idea where this wasted CPU time is going? It's not that expensive just to decide to cull everything, is it?

Thanks!