I see this is an old thread, but today I had a similar problem at a client with a Dell Celeron running Windows 10.

The CPU usage locked at 30% maximum utilization at 0.48Ghz, and the laptop was terribly slow.
It took 30 minutes just to open Outlook, to give you an idea...

I noted the following:
1. Although it was connected to power, the battery icon said: "Not Charging"
2. Battery level was at 1%
3. All battery settings for "Pressing the Power button", "Pressing the Sleep button" and "Closing the lid" was set to SLEEP

Client would close the lid at 5pm and put the laptop away, only to reconnect the next day. SLEEP mode drained the battery totally and since the battery power would then fall below 20% by the next morning, the Battery Saving setting in Windows 10 kicked in and FORCE the laptop to SUPER SLOW mode (locking CPU usage at 30% max).

This problem of SUPER slowness and CPU lock can thus only occur on LAPTOPS or devices with batteries in my view.

To fix this (temporary or permanently), do this:

1. Click the battery icon in the tray (next to the date, right bottom of the screen) - NB: NOT Right-Click
2. Click "Battery settings"
3. Un-tick "Turn battery saver on automatically if power falls below..."
4. Disable "Battey saver status until next charge"
5. Unplug the power cable and re-plug.

The CPU Usage should now go up to 100% again and the battery icon should say "Charging"

PS. Make sure your charger works and is the right type for your laptop.
Also, change the settings for "What the power button does" etc to Hibernate or Shutdown rather, if you know the laptop will be turned off for long periods of time to avoid draining the battery.