So after spent few times searching needles among the straws, I found this website that lists all PC Slow Down tools (now obviously that I didn’t key in the exact keyword). I decided to use CPU Grabber because this software is from DirectX Microsoft so I believe it won’t do much harm. Another good app is CPU Killer, but not a freeware.
CPU Grabber has small GUI with trackbar to set the percentage of CPU usage, and buttons to start/stop and close. So, if you need it either for playing old games, showing the slow PC to your parents so they will get you a new one, or running software validation in stressed environment like what I’m doing, just run number of copies of this program (number of your CPUs/cores) and set for each 70% or more.
So sorry this is being a geek posting, I hope it will boost up searching index of ‘How to Make Computer Slower’ in Google leads to the correct articles.