Make Cycles less greedy

In addition to being able to reduce the number of CPU threads that Cycles uses from within Blender's interface, you can also reduce its priority in your operating system's scheduler.

As for GPU, that's tougher to do, but I think nvidia provides a tool that could help with that.

If you want to keep your system usable while cycles is rendering then you have the option to assign the number of threads you want to devote to Cycles under the Performance tab in the properties panel (See attachment).

You'll now have some CPU overhead to perform other tasks on your system rather than grinding it to a halt, of course that will be dependent on the amount of cores/threads present in your CPU and how many you actually assign to the Cycles renderer.

In the case of a GPU, that's not possible so far with my limited knowledge, if you however have a better proposal than "A slider that limit the consumption of CPU/GPU" you're welcome to post the implementation here.

Your best bet for now is using a twin GPU setup, most likely a weaker GPU to power your Desktop while the stronger GPU is assigned to Cycles.

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I'm honestly not sure how possible this is. The problem is, generally you always need the whole scene in memory while doing the path tracing because there's a high probability that everything will be hit at least once. Or, at least that's my understanding. The other cost is computation, and I don't know if you can throttle that. Either way, I know that render engines like Redshift can render my machine useless if my computer is put on the farm at work, and the machine has a pretty decent GPU.

loool Make Cycles more efficient... Make Cycles less gridy... Haven't seen similar feature in any other renderer.

I'd suggest you to buy a better gpu. With 1080ti there's no serious lagging.