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I have new HP Elitebook 850 G5 laptop with Intel Core i7-8550U which has TurboBoost up to 4.00GHz and base frequency 1.80GHz.

However when i put the CPU under stress test firstly the frequency jump to 2.4GHz for 20~40 seconds and than falls to 1.40 GHz for the rest of the test.

MAX CPU Temp after running for 30 minutes: 63°C

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From How Intel Turbo Boost Works:

Turbo Boost is a dynamic feature. There is no set-in-stone speed which the Core i5 or i7 processor will reach when in Turbo Boost. Turbo Boost operates in 133Mhz increments and will scale up until it either reaches the maximum Turbo Boost allowed (which is determined by the model of processor) or the processor comes close to its maximum TDP.

Turbo Boost will only kick-in when one core is being stressed, since this will mean increasing power use for this one core, power which is taken away from the other cores. Use therefore a stress-test like Prime95 that can be run on one single core. Turbo Boost can only be obtained when just one core is being stressed and the CPU does not get too hot.

Turbo is autonomous. Don't expect it to keep maximum speed for any length of time. The point is to optimize resources in terms of energy and speed depending on the application. The algorithms involved are built into the CPU and cannot be controlled. Turbo Boost might kick in, just not the way you might expect it to look like. Sometimes it will boost for less than a second for a single set of instructions.

If you want it to always stay high, the only way is to disable Turbo and overclock the processor from the BIOS (not recommended).

Use CPU-Z to see Turbo Boost under the Clocks section in "Core Speed".

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  • So is it normal that after few seconds the frequency drops under base frequency and usage on all threads is around 75%?
    – Michalides
    Mar 16 '19 at 16:29
  • Yes, also the logic of the CPU Turbo Boost may sometimes conflict with Windows, if Windows decides to move the thread to another core. Turbo Boost works, but there are many variables not under your control. You can see with CPU-Z that the Core Speed changes repeatedly and rapidly.
    – harrymc
    Mar 16 '19 at 18:15

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