Issue sorted out! Anyone running an Asus board with this same problem - in bios CPU settings, disable CPU TM (Thermal management / Temperature monitoring). Number spiked immediately.
I wouldn't because you have no warning if your CPU is about to overheat.
If disabling thermal control "fixes" your issue, check your heatsink is properly mounted and your CPU is being cooled, otherwise the motherboard is throttling back your CPU to prevent it melting/burning/exploding/dying, hence the half-speed dial-back.