@ Prissi,
I can also say performance is slow on larger display. My computer specs are Core2Duo 2.93GHz, 3GB DDR2 RAM, ATI HD5450(1GB) with Win7. I usually play with a pak128 2048*2048 map with FPS 25. With a window of 1920*1080, having about 20 relatively small cities and 32 industry chains and 56 vehicles (mostly trains & buses), CPU usage is around 40% and GPU usage is around 12-15%. Changing the window size to 1280*768 changes the CPU usage to around 15-20% and GPU usage around 5-7%. Zooming in increases CPU usage. This means the CPU and GPU usage are both directly proportional to the number of tiles shown on screen. Using pak64 with same settings and window size, scrolling has huge lag and CPU usage is consistently higher than pak128. There were no other programs running simultaneously. This clearly determines that CPU usage is mainly for displaying/rendering.
In my opinion, more of the rendering should be offloaded to the GPU and CPU should be relatively free. If I remember correctly Simutrans stores all texture images and tile coords in RAM, but ideally it should first try to store it in GPU memory and if it fails then use RAM. Only the coords of the moving objects should be passed to GPU for rendering, rest of the static things to render should be handled by the GPU only. Most of the limiting aspects are due to CPU spikes especially while scrolling and otherwise consistently high. Its quite disapointing that in spite of a game which runs well on low config computers, simply a large window screws up the whole experience. I can't even imagine the pain that ojii must be experiencing with such a high-end config and still such a slow performance. Besides in today's date it is perfectly acceptable to lay down a minimum requirement of 512MB RAM and 256MB graphic memory, further subject to map size. As the game is still 2D there should be no major issues with onboard graphics or graphic cards.