[berkut]

It was the problem, because doing REYES hiding you suffer the huge "overdrawing" issue, where you need to set the shading rate per object in the scene based on its size on screen such that it doesn't get too much shading. If it had too much shading (the shading rate was too low), many more rays will be sent. And this is because REYES shades stuff before it sees if it's visible, whereas raytracing from the camera (with the raytrace hider in PRMan) only shades the points which are visible, so it scales much more on complex scenes (when GI comes into the picture - without GI REYES can cope easily as it pages everything).