I don't find a 27 fps average for something as simple as space blaster to be a good result. At all. On mobile: understandable, but on a 360? It's terrible, even if it's through a browser. It seems the performance should be better even if MS is too stubborn to support WebGL. Watch them come up with "DirectX: Live" soon, just to stay relevant.
Remember the XBox 360 hardware is from 2006. 6 year old hardware is pretty old by today's standards, and in fact modern mobile phones and tablets are probably actually comparable. So I think it is to be expected that XBox 360 performance is roughly what modern mobile performance is.