CPU not running at full clock speed ? Solution 100%

This happens most of times because of power issues,
you can fix this by physical actions like,
Replacing the battery or Charging adapter if battery has a good health.

But without doing these we can fix this by following these steps, here we set OS to run the pc on maximum performance and disable power management features and also we disable intel power management service.

First enter the BIOS menu - Press the power button to on the PC or LAPTOP and Press F2 or  F12 or DELETE button while the manufactures logo is shown.the button is different for different manufactures.so you can find it by make a google search.

Then Find the Intel SpeedStep Option from the bios menu this option appears differently in different bios version please check the images below so you can find it easily,if you have any issue please make a comment then i will help you.

Disable the Intel SpeedStep option then save the changes and restart the machine.

Then login to windows and go to control panel  > View by large icons > Power Options > Change
plan settings > Change advance power settings > Select High performance from drop down list >
Then apply the settings.

Now open the Task manager and check the clock speed.
Paste this link to the explorer then you can goto power options.

Control Panel\All Control Panel Items\Power Options