It seems to be really hard to improve the performance of a UIWebView, especially for websites like Mashable or Ars Technica, where tons of scripts are loaded and long, multi-pag
I don't really know if this can help or is accurate given the question, but here is what I have to say:
Some properties like shadow or gradient, and even images inside the WebView affects a lot the performances you can get. Try not to use them and/or to keep them as few as possible.