If I had to guess, the actual calculations for the logic of the game itself are completely trivial and are not bottlenecked by any modern processor or any number of cores. They're not exactly doing MCMC to simulate outcomes.
With zero knowledge of how it works, I would also expect that each tick is some trivial calculations to determine yield per square for each city (plains square starts at +1 * 3 workers * 1.2 improvement modifiers) and combat resolution. Deterministic calculations that should complete instantly.