CPU cannot run at full speed under Windows environment. However, the BIOS shows the correct specification.
Answer
For laptops powered by Intel Processor: This behavior is caused by disabling Intel Speedstep in the BIOS. Since disabling Intel Speedstep manage CPU to run at lower speed and under safe mode conditions. Therefore, if you want the cpu to run at maximum performance, you need to enable Intel Speedstep in BIOS and set the Windows Power Management to maximum performance for both AC and Battery mode.
For laptops powered by AMD processor: This behavior is normal and caused by the Power-Saving technology called AMD PowerNow which automatically switches the system between maximum performance mode and battery-optimized mode - its main function is to automatically adjust the cpu performance accordingly to the application being run. If you wish to have the maximum performance, then you are advised to disable AMD PowerNow technology located in the BIOS setup menu. However, having the AMD Power Now disabled in battery mode will shorten battery life.