Use cputhrottle command-line application.
Download from http://www.willnolan.com/cputhrottle/cputhrottle.html
renamed the file by the following command:
mv cputhrottle.gz cputhrottle
Then it is time to set the execution permission for this file:
chmod +x cputhrottle
sudo ./cputhrottle 12789 25
So in the command above, the first number is the PID pf your desired process and the second number is the percentage you want this process to use.
The only step left is typing your administrative password because you used sudo command.