When that happened to me I figured later it was a driver *issue *(there’s also something bout it on this thread). NVidia drivers will prioritize any 3D work over CUDA processing. So minimize Unreal while building, if your browser uses hardware acceleration minimize it too or turn that off and, in my case, I have to close Wallpaper Engine too. ***Anything ***that might be using the GPU for graphics processing, shut it down or minimize it. I saw an instant speed difference after closing/minimizing those while GPU Lightmass does its thing.