If I may suggest - Regardless of how many frames per second that the game is rendering, the game itself is still only running 40 ticks per second, no more. My honest advice would be to not unlimit the FPS, since that simply increases your CPU/GPU load with no actual benefits. Ideally, for your configured setup, you want the game running at 40 FPS - One Frame per Tick, as the game originally ran. The only reason we have more FPS now, is because we are tweening the animations from one tick to the next. (Think Tween from Adobe Flash - we're filling in the gaps.)
Minimizing the game also helps, since the game basically stops rendering, while not running in headless mode.