It's normal for clock speed to drop if processor is not under load it will lower to save energy, with Windows power setting @ default Balanced plan,.. with set on High Performance you will run at your BIOS setting 24/7 regardless of load.
Another thing that will lower clock speed would be your off-set for AVX, processor will throttle down cores/threads when running heavy AVX workloads like Prime95 depending on your BIOS setting.
While I suspect this is an Intel CPU (is it the 8086K in the signature?) I know with at least the Ryzen CPUs from AMD the clock speeds will still change, even with Windows' High Performance power plan. As I recall, this is the chip itself managing its states to reduce power usage when not needed. (A special Balanced power plan needed to be made then because of conflicts between the chip and Windows both trying to switch states, with the chip being able to do so much faster.)
Unsure if Intel processors have a similar capability, but unless the chip is throttling due to temperatures, variability in speed should not be an issue, but also nothing is wrong with using the High Performance plan.
Processor: AMD Ryzen Threadripper 1950X @ 3.8 GHz
Cooling: Corsair H110 AIO
Motherboard: ASUS Zenith Extreme
GPU: AMD RX Vega 64 (Stock @ 0.965 V +50% Power Limit)
Memory: G.Skill TridentZ 4x8 GB (32 GB) @ 3200 MHz 14-14-14-28
PSU: OCZ Fata1ty 750 W
OS: Windows 10 Pro 64-bit
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