[html5] Dragging an image across a canvas

[J David Eisenberg]

At http://langintro.com/metropath5/test.html, I have a demo of an
image that is larger than the canvas; you can drag to see portions of
it. It works great and pans smoothly.

If you click the "-" icon at the lower right of the image, the image
is reduced (by setting a multiplication/divison factor between the
image and the canvas). In Firefox 3.6.4 on Fedora 13 Linux, when I
drag the smaller map, the image does not update until I release the
mouse. Using Google chrome on the same system, it still scrolls
smoothly.

I'm guessing this a JavaScript performance issue, but I may also be
missing something in the algorithm. Comments/opinions welcomed.