The original cities skylines had atrocious performance after a certain city size as well, especially on hardware available at launch. Hopefully they can deliver on better performance over the next few months, I expect it to improve before they do their console release.
Cities Skylines 2 reportedly runs with 7-12fps on an Intel Core i9 13900KS with AMD Radeon RX 7900XTX at 4K/High Settings (www.dsogaming.com)