I did some testing around AMD's and Nvidia's performance with OpenGL. It's a known fact that AMD's performance with OpenGL is pretty terrible, and as it turned out, that's the root of my performance problem, and probably for other current and future players with similar setups, 'cause I don't believe that I'm the only one with this issue.
My rig is a Core i5 3470 with 16 gigs of RAM and a Radeon R9 380. It's been serving me well for many years now and still chews through everything I throw at it like a champ. Except for this game and Cattle and Crops. The similar thing about those two titles is that both are running on an OpenGL-based engine, and both games have performance problems with AMD graphics cards. The dev of C&C even made a post about his experiences on the AMD forums. I had similar results with LOTUS.
As a comparison I used a GTX 960 from my brother's PC. Both graphics cards have 4 gigs of VRAM, I was using the latest drivers with the only tweak being forced 4x anti-aliasing in the graphics driver. My test run was made on the test map on the "Best graphics" preset at 12:00.
This is the run with the Nvidia card. The fps moved mostly in the 50s, sometimes reaching the 60 fps cap. The game was running smoothly overall, minus the micro stuttering.
And this is the run with the AMD card. The fps was mostly in the 30s, rarely going above 40, sometimes dropping below 30, especially around the cobblestone areas. As you can see the GPU here is running on lower memory and clock speeds, with lower utilization.
Side note: I also went through the performance entry of the Lexicon, even loaded the test-egg map with the test train, the fps just wouldn't go over 37 with my Radeon.
Since OpenGL is pretty much a deprecated API, and the products of one of the two available graphics chip makers are performing worse with it, than the competition, wouldn't it make sense to port the game to Vulkan, especially during the EA phase, when the game is still being shaped? I'm really curious, because right now LOTUS is running worse than OMSI, and that's not what was promised. Any plans regarding this?