Performance is far worse because this is the equivalent of a procedural texture. With a tiling bitmap the computer simply fetches the file, then paints the pixels into a region.
The browser first parses the CSS, then generates the actual bitmap data for this definition in memory then finally paints the pixels into the correct places. The in-memory bitmap generation is far more taxing than simply tiling an existing bitmap.
Then there is the issue of dynamic CSS which may cause reflow which will in turn probably require more dynamic bitmap generation.
One interesting thing is done while building iOS app: png images are reencoded to make them more efficient. What is done, that alpha value is premultiplied, but the more interesting thins is that bytes are rearranged from RGB to BGR . This is done to match the layout of the iPhone display, so mapping from image is even faster.
