The key to Akihabara performance is small number of actual pixels being drawn/computed.
Resolution of game's <canvas> is just 320x240 (check "view image" in Firefox) which is then blown up by browser renderer to 640x480 (in a same way how you can resize image in CSS).
Thus you get 4x as many pixels and cool retro 8-bit look with a lot of work done by browser's C++ code instead of much slower JavaScript.
Till we don't have hardware acceleration for <canvas> tag, unfortunately demos and games are going to be quite limited.
