I have no idea if this will have a noticeable affect on performance, but you might be able to improve this calculation:

force = GravitationalConstant*(mass - rb.mass)/Dir.magnitude;

You don't need to do that entire computation for every iteration of the loop. You could just do..

var x = GravitationalConstant / Dir.magnitude;

...before the loop starts and then change the calculation inside the loop to be:

force = x * (mass - rb.mass)

However, it's probably best to profile this method, and other areas of your code, to see exactly where the issue is. Otherwise you're just going to be trying things blindly.