Bear in mind that this is inverse square root (a misnomer; it's really 1/sqrt, or the reciprocal), not just the square root. Square roots are decently fast as

Bear in mind that this is inverse square root (a misnomer; it's really 1/sqrt, or the reciprocal), not just the square root. Square roots are decently fast as provided by the processor, but not when combined with the division needed for the reciprocal; combining the two is why FastInvSqrt is so fast.