CPUBalance intelligently optimises processor use to keep your PC responsive, even when under load.
(Sceptical? We would be too, normally, but the program comes from the developer of the popular Process Lasso, and uses some of the same technology, so don't rule it out just yet.)
The program requires installation, but after that you don't have to touch it at all. We used HeavyLoad to max out our system, which would normally bring it to a full stop, but CPUBalance stepped in, lowered HeavyLoad's priority, and we could keep working as usual.
If you need more control, a ProBalance Settings dialog enables fine-tuning operations. You could have the program start working earlier, or later; include or exclude foreground processes; set CPU affinity during restraint (allow resource hogs to use specific cores only); lower I/O priority of target processes, and the list goes on.
CPUBalance works entirely transparently. There are no alerts when it restrains a process, and all you're likely to notice is that your system is more responsive than before.
The program does offer one helpful extra, though, in a list of its most frequently-restrained processes. If you see the same names cropping up regularly, do some research to see if they can be tweaked to reduce system load.