How to fix severe frame drops in Cities Skylines II with high-pop cities?

After spending dozens of hours building a super-city, the frame rate crashed to 12 FPS once the population hit a million. It was literally a slideshow. I tried adding a 32GB page file to the system, but while it stopped the crashes, the frame drops actually became more frequent. I then used the GamePP Resource Scheduler to set the game priority to Realtime and capped background GPU usage. Frame times were suppressed from 80ms peaks to 30-45ms per RTSS, with GPU temps at 75-82℃ per GPU-Z. It's finally playable.