I've just been playing with my entry, wondering why so many people are finding it slow. Much to surprise, I've found that if I _increase_ the canvas resolution from 16 to 32 bits, it runs much smoother.
Now DON'T LOOK AT THIS UNTIL YOU'VE JUDGED THE UPLOADED ENTRY. But if you want to see a much smoother entry, where the only difference in the code is the increased screen depth, a new version can be found here. It is a better game for just that little switch. Ho hum. I'll know next time.