Reboot into Safe Mode. Run MSCONFIG from there. Once you have disabled all of the non-Microsoft services, you can reboot into normal mode. I suspect that you will now have regained control of your computer. If so, then you know that one (or more) of the non-Microsoft services is the culprit.
My sister-in-law had exactly the same situation as you; the only difference was that she had Windows 10, and you have Windows 8.1. But her computer was slower than molasses, so slow as to be unusable. I restarted the computer in Safe Mode and I was able to go into MSCONFIG and disable all of her non-Microsoft services. This eliminated the problem of extreme slowness. I then re-enabled one service at a time, so that I could see which one was the culprit. I kept that one disabled, and re-enabled all others. Her computer operated normally after that.
Here is a tutorial explaining how to start Windows 8.1 in Safe Mode:
Safe Mode - Start Windows 8 in