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To put things into perspective, the Microsoft Surface Pro 6 with its lowest spec i5-8250U runs at a base frequency of 1.6Ghz while an i5-8350U at 1.7Ghz powers the lowest spec Surface Book 2 15", so it can be quite frustrating if your device suddenly drops to a quarter of its processing speed.
That's what recent user reports have indicated where their "ultra-portables are throttling the processor down to 400 MHz, a state that—in some instances—persists across reboots," says TechRepublic.
The issue apparently, has to do with processor throttling that's erroneously kicking in by setting an Intel CPU flag called BD PROCHOT (bi-directional process hot). The flag can be set by any peripheral and is a safety feature designed to protect the system from overheating. Although the CPU could be operating within its thermal limits, this flag can be set if one or more components linked to the CPU are at the limit of crossing theirs.
The publication also notes that while BD PROCHOT is not unique to Surface devices, the recent emergence of user reports around these systems is indicative of a correlation at minimum. "We are aware of some customers reporting a scenario with their Surface Books where CPU speeds are slowed," said a Microsoft spokesperson, and that the company is "quickly working to address via firmware update."
It still remains unclear what exactly is triggering this throttle-lock. Some users report that the issue is resolved after disconnecting the Surface Dock or AC adapter while others blame a third-party antivirus software behind the slowdown. Whatever maybe the cause, affected owners of these devices should soon expect a firmware fix from Microsoft.