Maybe it was during the initial "generating textures" phase when you first launch it, but it should definitely not be lagging at the menu.

I'm on a 2080 S + 9700k and the game runs pretty decently, small city and it averages maybe 45fps with most settings on high at 1440p, run over 70 fps if I turn down the settings a bit, but I like the graphics and I can live with lower FPS on a city builder game.