It's a cache of QQuickPixmap rather than QPixmap. I should have been clearer. Probably best to read the code I linked at gitorious. To create a texture from a QPixmap, it is first converted to a QImage. The rest of the code is basically identical, and it's cached the same way (what is cached is QQuickPixmapData which consists of a texture factory for the image data). In short, two requests for the same QQuickPixmap (ie, see QQuickImage for an example) won't need to go through the provider->requestImage() or provider->requestPixmap() codepath; instead, the data will be returned directly from the cache.
Performance should be mostly identical, I guess. Benchmark it to find out for yourself though, as I may be wrong about that. Certainly pixmap.toImage() must have some overhead.
What is eventually rendered is a texture, not a pixmap or an image, of course.
Cheers,
Chris.