That's because at 1920.1080 the load is mostly on the gpu. At lower resolutions the load is more on the cpu.
Indeed. However, at a lower resolution, the FPS will still be increased
, even if there is now a CPU bottleneck. It will bottleneck at a higher FPS than it would at a higher resolution.
At 1280x720, At 1.25GHz 2 cores only, average FPS was 83FPS, up from 73 for 1920x1080. However, at 0.75GHz dual core, it was the same at average 50FPS. 3.25GHz quad got me to 123FPS.