why bigger cities and LOD setting are having such a huge impact on performance - the answer is - pedestriansare killing the performance - disabling them via developers mode can give even more than 100% fps increase based on your LOD settings!
https://www.reddit.com/r/CitiesSkylines/comments/17julc6/why_bigger_cities_and_lod_setting_are_having_such/
the LOD settings gives huge boost because... on very low (25%) the pedestrians are rendered only when you zoom in - but higher LOD settings make them visible even from far away and it absolutely tanks performance - I play on RTX4090 in 4k and have set LOD to very low (25%) because otherwise I get 20 FPS even zoomed out. After disabling cims even on High LOD settings (70%) I got 50+ FPS and the game (especially vegetations) looks much better.