Well it's two issues because they are coming from two different aspects of the PC, and they have two different effects.
The issue on simulator speed is coming from CPU load due to simulation calculations, from what I have seen on my PC and from what you can read around.
The issue on FPS is due to the graphics and, if I am not mistaken, partially a choice from the developer to ensure a smooth experience rather than going for FPS for no reason.
Comparing a few things, the entire situation is just odd and it looks like they intend to use dlss/fsr? as a crutch.
Main menu - seriously, why is it hitting the resources so hard for a photoshopped screenshot and a few buttons.
Play with the maximum frame latency setting in graphic options and compare things such as:
1) MFL: 1, Game paused, zoomed into the ground with nothing in sight
GPU at 50-60% usage and high cpu, but the game is paused
2) MFL: 3, Game paused, zoomed into the ground with nothing in sight
GPU works overtime 100%, probably discarding the queued frames because again, the cpu is being hammered, maybe a few percent more than (1).
3) MFL: 1, Game running, zoomed into the ground with nothing in sight
Same as 1) but ~5 fps less approx. due to simulation.
4) MFL: 3, Game running, zoomed into the ground with nothing in sight
Same as 2) but ~5 fps less approx. due to simulation.
5) MFL: 1, Game running, zoomed out overlooking a city of say 100k.
6) MFL: 3, Game running, zoomed into the ground with nothing in sight
etc etc..
I don't have all the correct figures to hand, but I did this as an observation for myself
The way I see it is MFL queues frames but clearly can't deliver (hence 100% vs 50-60% usage) and is wasting resources because perhaps the UI system is eating cpu resources (main menu is??) and taking it away from simulation and rendering pipelines. I don't know, I'm not an expert, but a paused simulation vs a running one, should have a very different picture, and it doesn't seem to have a major effect up to a point, obviously 1mill pop would be different!
I'm sure they will figure it out, but there's clearly more going on than just simulation / pathfinding etc, when the game suffers just as much paused as it does running.
i9, 14900k & RTX4090 with 64gb of Ram and the game struggles with low pop counts for me.