I know youve checked this but just make sure some program hasnt changed it and type Power Options into windows search bar and open it up, next to the play you have selected (which should be balanced) hit "change plan settings". Then hit "Restore Default settings" for this plan, then to double check its worked hit "Change advanced power settings", open "Processor power management" and check "Minimum processor state is set to 5%"
If all that is correct you could check that the windows power system is actually working by changing "Maximum processor state" to 50% and save it then check your clock speed, it should drop it down. Mine drops to 1900mhz on 50%.