It was great fun watching the buildings get destroyed but it did lag terribly when it happened. The movement felt a bit slow but the mouse sensitivity seemed really high, but some of that was probably down to the lagging after hitting a building. Try breaking the buildings into larger or different sized blocks, a building made up of 10x10x10 cubes with a size of 1 is 1000 rigid bodies all created in an instance, where as the same building made up of 5x5x5 cubes with a size of 2 is only 125. What you could then do is have some of the larger cubes break again under high impact creating a similar effect with a bit more variety without maxing out the CPU.