Quicker way to calculate geographic distance between two points

I borrowed the following method from somewhere on the internet (Can't remember where). But its doing a straight forward process, finding the distance between two gps points. It works just fine, except that it may be a little slow, as I'm running it across millions of points. I was wondering if anyone knows an approach that would be computationally less expensive.

The accuracy needs to be in the general area of 'correct' but doesn't need to be 100% accurate.

private double distFrom(double lat1, double lng1, double lat2, double lng2) {
    double earthRadius = 3958.75;
    double dLat = Math.toRadians(lat2-lat1);
    double dLng = Math.toRadians(lng2-lng1);
    double a = Math.sin(dLat/2) * Math.sin(dLat/2) +
           Math.cos(Math.toRadians(lat1)) * Math.cos(Math.toRadians(lat2)) *
           Math.sin(dLng/2) * Math.sin(dLng/2);
    double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
    return   earthRadius * c;
  }
}

P.s I did indeed find a number of other relevant questions, but they don't really focus on my speed concern.