Bad engine, bad optimisation or both. Don't spend crazy amounts of money on kit with Stellaris's performance in mind, you'd pay so much for so little gain. I've got an £8k gaming rig, and struggle to push Stellaris past 130fps @ 1440p. (Which goes down to 1fps at the turn of each month late game.) Conversely, I can play the likes of Doom Eternal and Death Stranding at nearly 200fps.
As a matter of interest, why in Stellaris which is not an FPS/TPS or RT action game, do you need more than 60fps? I am asking so I can understand your point of view.
If there's no display update required because at the end of the month, you're zoomed out or looking at a quiet system waiting for the new resource numbers, why is slowing to 1fps a problem? (that's what an efficient graphics driver ought to do, the screen displays an unchanging frame buffer with no cursor movements. Are you watching rapidly moving objects? Because I am not, just the numbers at the top of the screen.
Honestly I actually can't see much Galaxy map benefit from 3D, effectively it's a 2D map; but it's a market acceptance thing - gamers expect and think they need 3D objects. Even changing view for eyecandy doesn't really turn Stellaris into a genuine 3D game as you just select and click on objects and points; then watch the ships go in straight lines to stationary fixed orbits on flat planes. It actually irks me, that the game is so inefficient graphically, wasting energy. Games with actually 3D world modelling allow a 30fps setting for weaker systems and I'd be very happy to use that in Stellaris as it's simply not a rapid reaction twitch game with movement responding to constant inputs, but a game where you queue up orders then watch what happens. For instance in Total War, commands get laggy in older games at HD resolutions making it difficult to give orders. But it was only a gameplay problem at about 1/10th the FPS you are seeing. I would prefer more "friction" changing to system view from galaxy and fighting out more of a realistic battle non-arcade style battle. As is you can pre-order some flanking maneuvre from the neighbouring system when you have multiple fleets engaging; though I confess at times I've had debacles through mis-timing the fleet coordination.
Frankly, if it's to watch big kick ass battles close up, I'm finding the fleet conflicts so lacking in battle strategy compared to Total War, that I often miss them or watch zoomed out, monitoring the battle summary popup. For me Stellaris is less 3D immersive than the 8bit Mosfet 6502 based game Elite, which wowed with 3D wireframe graphics in the early 80's giving a system where you appeared to control your ship through space (but just without accurate physics the ships were controlled more like planes via rolls and banked turns).
This is a grand strategy game, where it's the decisions, not the fine motor control and rapid reactions which make play sucessful or not.
From what I've heard, simple mods in past, helped much late game lag, simply turning pop migration into a monthly decision.
