Do you have enhanced sync / anti-lag enabled? Do you run any overlays in background (e.g. steam overlay or performance monitoring)? Have you tried disabling every disablable stuff in "Windows control panel" - "Gaming"?
Diablo III: 180 fps drops and lags down to 15 fps
Overwatch: 400fps to 25 fps and lags
GPU clock goes to about 500-1200ish
GPU memory clock stays around 2130+-
gpu temps are fine.
CPU never really goes above 3-10% almost no load really...