when you say which is the culprit? how will i know how much cpu a program should use? thanks
System idle process should typically use the most. (It's actually not doing anything, but shows how much CPU is available.)
Other processes use a few % at times, when the computer is running them. If any program constantly uses more than about 5% or 10%, time to start looking at why, in my experience.
It's not unusual for some programs to briefly use almost all of it, (>80-90%) very briefly, while starting or performing labour intensive operations.