I recently had a Lenovo Thinkpad T440P with this problem. Running Windows 10 Pro, had really slow performance, and the CPU was stuck into this speed, unable to throttle up.
How to fix
Go into the BIOS and disable all settings related to CPU performance related to power management, including SpeedStep. After disabling this settings, computer now runs at maximum speed all the time.
Cause
In this case, the laptop had a dead battery. This forced the power management to keep the CPU at the minimum possible frequency.
Other effects
Unable to update BIOS due to battery unable to charge.
