-threads <number of cores/threads>
I was not able to find a definitive answer about the maximum number of threads that CS:GO uses. If you have a CPU with 4 or more cores, you can try to set -threads to your cores (or number of threads, but I can not guarantee that your performance will improve. Test it, if you don’t notice any difference or your performance is actually worse, remove the launchoption again.
Valves server software on the other hand is only single threaded.