Guessing: rendering anti-aliased text over an image background is expensive (you have to read the background image to blend colors). Re-rendering page text multiple times while an image is loading may not be a good idea.

Browsers seem to be very lazy about this kind of thing, perhaps for that reason. Chrome dealt very weirdly with one of my sites, where I had an image, with a transparent-background iframe containing a semi-transparent-background div above it, and text above that iframe. The text's position was lagging relative to some images placed above it. I wonder if this is why.

Right?! This is very interesting and I just discovered it by chance. If it were just one browser we could write it off, but it's not. I'll try to find out.