Thank you for your feedback! And yes, I did use an old newspaper for the hit effect C:
And yes you're right, it's hard to see, I've just added this to the issue board to be solved!
Performance is a known issue, the background update rate gets reduced for a certain amount to at least keep the game a little bit more playable, but you can try reducing the window size for now as that reduces the amount of background update-calculations. Future versions will have more performant algorithms as well as offload rendering using the WebWorker.