I'm not a C4d user and I have no experience on this software, however many people in 3ds Max side do suffer from the same frustrating experiences. What I often tell them is, 25Million polygons is nothing for a graphics card to actually "shade" the scene, but it is the topology modifiers or sorts of calculations "which depend on the CPU", causing these lags.
You can benefit from your GPU on applications like Lumion or during the rendering of GPU renderers like Iray, Vray GPU, Octane etc.
Most parts of these software don't even benefit the patented technologies of the CPU's, with glamorously shining names and still rely on the raw CPU power.