Just wondering if it could be power or heat related. Or to be more exact, thermal throttling. The 1080Ti being a powerful factory overclocked card will also produce more heat which may cause it to start protecting itself by lowering voltages and clock rates. It could be something that simple.
Underclocking your GPU might help keeping the temperatures low, thus allowing it to maintain the highest possible performance for longer periods. Of course the peak performance will be lower but the average FPS may be higher.