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bbfa235
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(Get the code!)
After a bit more digging I discovered something interesting. When idling, the CPU has the full range of frequencies available to it, from min to max. But, when the load ramps up, the BIOS appears to step in and limit the frequency, then ACPI throttling steps in.
I used a simple script (attached) to discover this. Starting from an idle, cool CPU the BIOS maximum frequency is at max. A few seconds later, the limit starts to drop. A few seconds after bottoming out, ACPI throttling begins to increase. The full output I get is appended to the (in comments, at the bottom).
It is also interesting to note that the CPU fan is always at max, even when the CPU is idling.