All right, thanks to the marvels of Web Workers, I now have zones loading in the background smoothly & transparently. It’s smooth as butter.
In the top-right is a debug-indicator, showing which zones are cached (green), not yet loaded (yellow), and loaded in the background (purple.) If the processing fell behind, you might see blue, which would be ‘loaded in serial’.
Since I successfully implemented the parallel processing approach, I have not yet seen blue!