I am not implying, I am saying it clearly. I know it sounds crazy but 1080 and 1080ti are so fast that the processor can not feed the GPU with the necessary info fast enough at 1080p / 60Hz. For other resolutions and / or higher Hz, the GPU starts to have difficulties and the CPU slows down.
There are plenty of threads regarding this issue. Search the forum and you will understand better from someone that explains it with proper English!