model: MacBook Pro 15-inch 2018
macOS: Mojave 10.14.5
CPU: Intel i7-8850H

Recently, I've observed a significant slow-down of the macOS during high disk I/O. Sometimes the CPU frequency would be locked at 800 MHz, making everything unresponsive. An example is shown below

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The fan policy appears to be different and more aggressive. Recently macOS would kick-off the full-speed fan even when the CPU frequency is around the base-clock-speed (2.6 GHz) and the CPU temperature is barely 70 C.

Would anyone be able to explain what's happening behind the scene? Why lower the CPU frequency so much during high disk I/O (I don't recall a similar polity in the previous macOS)? And why the fan policy became so aggressive recently? Thanks.

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