After few tests I found out the more skinned mesh renderers you have the more method Rendering.UpdateDirtyRenderers cost your cpu power.

Project Steps:

1. Open scene "Test". Scene contains 270*6 objects and only 270 of them are visible.
2. Open profile and find Rendering.UpdateDirtyRenderers (Its probably in the top).
So, excluding vsync, this method every frame takes more then half my cpu power.

I noticed that this behaviour is not regression and it leads to believe that its by design. But if you ask me its extremly unoptimaized, there is no screen cullling for this method => The more skinned mesh renderers you have in the scene the more this method will consume your cpu. Also naming is quite confusing because UpdateDirty shouldn't be called every frame, so atleast rename it to Rendering.UpdateRenderers, because not it acts like that.
Any way, if its intended could you explain the behaviour behind thie method?