Linux is much better at multitasking processor-intensive tasks than Windows. I remember how virus scanning used to make by old Windows PC almost unusable. Linux is much better, but sometimes bad things happens ! Perhaps a plugin of your browser is using all the CPU, or some bad software is freezing your system, or Apache it’s eating up all your resources on your server.
If you have seen some situation like these, don’t worry anymore, you don’t need to kill all the offending processes, restart your graphical session, or even worse restart the computer, you can simply put the specific process in “PAUSE” and analyse the situation, in some cases you could find the cause of a poor performing process, or just restart it in a second moment, maybe after you have saved all your works, when you can give to that process all the CPU.