Browsers give a low priority to image refreshing, so in the time your code is manipulating/inserting in the DOM, the browser is busy with that and doesn't have time to repaint the image.
There's not a whole lot you can do, besides optimizing your code so that the processing you're doing with the ajax data is less intensive, or for example if you're getting a list of 1000 items, insert them in the page in intervals of 50, with a small delay between each, so the browser has time to repaint.
YMMV, maybe it looks great as is in Chrome, but freezes for 5 seconds in IE.
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