It appears as though a recent firmware update is causing Surface Pro 6 and Surface Book 2 devices to heavily throttle the processor to well below where it should be running. Multiple users who own one or the other device have reported seeing their CPU speeds stuck at 400MHz, and in some cases, the throttled state persists after a system reboot.
There have been similar reports of this type of behavior in the past. However, reports have increased in frequency since Microsoft rolled out firmware update two weeks ago.
After doing some digging, users discovered a link between the aggressive throttling and an Intel CPU flag called BD PROCHOT (bi-directional processor hot). This can be set by any peripheral, and it instructs the processor to run at a lower frequency to keep CPU temps in check.
This has reared its ugly head in past, as components other than the CPU can cause the processor to throttle-lock at a slower frequency. It's not entirely clear if BD PROCHOT is the culprit (or part of the culprit) affecting Surface Pro 6 and Surface Book 2 systems, but something is definitely afoot. Microsoft acknowledged as much in a statement to TechRepublic.
"We are aware of some customers reporting a scenario with their Surface Books where CPU speeds are slowed. We are quickly working to address via a firmware update," Microsoft said.
In the meantime, there is a third-party application called ThrottleStop that might help. It is designed to stop CPU throttle locking, and if a system is actually getting too hot, it initiates a shut down. We have not tested this utility ourselves, so use at your own risk.