The open source tool DxWND can be used to play (mostly older) games in windows mode and has also other tweaks to let games run better on a modern machine. Does anybody here use it and also reduces the demand for cpu cycles? It runs the game I use (Pacific General from GOG) but one hyperthread under full lot and that's annoying because the fan has to do some work. There is a 'Timing' tab, but both frame limiting and time stretching doesn't bring the cpu level down? Has anybody ideas how to achieve this without slowing down all hyperthreads or other programs and manually identifying a thread and configuring it? Thank you.

You could try the slowdown tool cpukiller which supports Hyperthreading.

It already uses only one hyperthread. Quote from the web site: So the total CPU load would remain, that's not what I am looking for.

I know, but still worth a try, since occupancy doesn't necessarily mean load. You can occupy resources without stressing the components. At least that's how I would have implemented it, otherwise it'd be rubbish.