You should change the payload mode so that the weight is not on a chain. at some of the higher tier rockets with heavier weights, the weight bouncing around on the chain starts to cause massive lag, which a flash game should not be causing for my beast of a computer.
No lag for me when using high tier rockets, just bounces around. Which admittedly, is annoying. But this game never made my computer lag. And I'm using a cheap computer at work.