Ads mean iframes, which mean tons of separate DOMs, which are heavy and expensive. You also have to consider that images are decompressed for display; that 474

Ads mean iframes, which mean tons of separate DOMs, which are heavy and expensive. You also have to consider that images are decompressed for display; that 474x350 image in the lead spot on the Techcrunch homepage is 474 * 359 * 4 = 663,600 bytes in RAM, plus any overhead. When you consider how image-heavy sites have become, that's a lot of memory that can disappear in a hurry.