get gpu-z and watch the sensors tab
if the vddc current red bar shows lines or gaps it's throttling.
Watch the VRM temperatures ( all of vddc phase #X temp ). The core temp is insignificant, as the VRM will overheat long before the cores do.
the VRM need to be below 125C to prevent a throttle. If you see temps above ~110c it's likely throttling before you see the temp spike. Best bet it to keep the vrm below 100C