well it actually uses 3-4 cores, but some parts of the client might be limited to one core, maybe.
the ingame ping is not only issued by the network connection, when the client starts to getting bottlenecked because of the issues above, it also starts to become unable to handle the data traffic in time, which causes increase of ping (too).
But something else, for Ryzen CPU in chiplet design you should care about RAM speed, because this clock also may limit the communication between the chiplets (and Cores) of your CPU.