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

Source: Hot Hardware – Surface Pro 6 And Surface Book 2 Are Throttling To 400MHz, Microsoft Working On A Fix