OpenTTD isn't supposed to be heavy on the graphics hardware in the PC, basically all graphics rendering is handled on CPU and the finished picture is then copied to screen as a single surface. (The common rendering process for 3D accelerated games is that the game sends commands to the GPU about the geometry to render.)
A little while ago I uploaded a testing build on the issue tracker for someone else who is also having (different) performance problems, this test build has a new FPS meter feature. Post on issue tracker
, Direct download link
If you unpack and run this version (don't overwrite your current install!) you can try opening the frame rate window and check the Graphics Rendering and Video Output timing numbers. It's common for graphics rendering to get slow-ish for complex scenes with lots of networks and in high resolution, but the video output should generally stay below 1 ms all the time on a modern system, regardless of resolution.
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