If you are using VRayProxy objects, these will be loaded from disk while V-Ray is rendering; obviously this process cannot be multithreaded (the hdd is only one, after all).
Also, you can try to increase the dynamic memory limit value - if it is too low, V-Ray will be forced to remove and re-load geometry too often.
hmm so only improvment in my case is make quicker hdd transfer to ram ? and my memory limit is set to auto , in task manager i see consumed for max around 10 gb of ram …