this definitely isn't the case in my experience. I run Civ VI on a relatively high end desktop (5950x + 3080 Ti) and there is a very noticeable slowdown between turns with lots of Civs/city states on large maps
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.