Advanced lighting with shadows own take serios CPU power. When you shift and the shadows cascade, and you are not using a I& or I( with serious single thread speed. , it will dip. Sunset does cause a heavier load (and some other simular) but the hit should quickly even out. The textures and shadowing may be hindered by the lower GPU, see if you can force setting in NVidia to utilize the CPU rather than GPU for offsets.