At this writing, NVIDIA has been really late in providing Drivers for NVIDIA cards in High Sierra up to 10.13.3. The provided Drives are very slow compared to drivers in previous versions. You cannot accurately evaluate graphics performance until MacOS is providing better support for NVIDIA cards.
Some Readers got a little bit better results by downloading CUDA and Drivers from NVIDIA. Many say these are a little better, but still not up to par with Sierra and previous versions.