Swarm is very a very CPU intensive process. If you have a complex scene it’ll use more calculations and RAM.
So more than likely, depending on the scene and the amount of RAM you have this is why your computer may feel sluggish or unresponsive. As for CPU cores being used, if you’ve got a Dual Core processor Swarm Agent will leave a single core free for other tasks or operations, whereas if you have 4 or more cores Swarm Agent will leave 2 cores free for tasks or operations. This is still predicated that you have enough RAM free to perform normal operations though.