Does anyone else experience a drop in FPS if you enlarge the game window beyond, say, 1920x1080?
If I enable the game's FPS counter I see graphics cap out at 60 (50) FPS regardless of init config (I capped game FPS to 60 to avoid excessive update rates though).
If I enalrge the window to 5120x1440 the same save has 60 (~25) FPS. with UI FPS also the game's responsiveness drops and key inputs and mouse presses are lagging.
Fullscreen and windowed mode doesnt make a difference.
1920x1080 is about 2 megapixels; 5120x1440 is about 7.4 megapixels. The computer has to do nearly four times as much work to put things on that larger screen / window, so if all other things were equal you'd expect about 1/3 to 1/4 the update speed. Additionally, IIRC the brothers have not had a large or high res screen until very recently (last few months), so there has been little testing of the performance impact.
Putnam's SDL2 improvements may bring display improvements of several sorts, and it's more likely that things will be tested for higher resolutions. That said, on the most recent (Jan. 2023) Steam hardware survey, about 77% of gamers were still at 1920x1080 or less; and only about 6.5% were at higher than 2560x1440 (~3.7 megapixels), so having twice that is still a bit unusual.
The main thing that really needs improvement is better decoupling of simulation speed, display speed, and user interface response speed.