Not the GP, but Civ is CPU bottlenecked. I don't know how their code is written under the hood, but games slow down as you get farther in the game. If you play too many turns the game can take minutes to process a single turn.

I can saturate all my 16 cores, on my workstation, the only result is that the turns are processed faster. When you play multiplayer, everyone is going to be pinned to the slowest person in the lobby.