I get 30fps with a 1x1 grid, when the simulation is empty. Randomly using air and wind, putting a bunch of plut and neut, etc. brought it down to 20. My laptop isn't the best, but it's able to run games like Space Engineers without too much difficulty, so I'm sure other people could get anything from 10 to 45ish fps, depending on their computers.
Also, for the record, making a version with a 1x1 grid is dead easy, as long as you know how to compile. You just find the value of cell size in one of the scripts and change 4 to 1, if memory serves
I seriously doubt that it would slow the game. The value would be a single machine word (dword on 32 bits, qword on 64 bits) and would almost definitely remain in cache at all times, if not in a register (especially on x86_64, where we have the luxury of many general purpouse registers).
Would you at least look at a PR if I were to submit one?