Every element that can be obstructed needs to check for being obstructed.
Seeing as you have well in excess of 1000 such elements, it is no wonder that the PC can't keep up with the checks.
Whilst the game probably has room for optimization in the performance regard (who woulda thunk it), you got to ask yourself, do you really need all 600 of those adjustors? There comes a point where the player becomes the optimization bottleneck.
From my observation fuel ticks down every 3s or so, however you shouldn't be moving or accelerating slower just because you have a lower framerate. At least in theory anyway.