The cache buffer refers to number of (measured in vertical pixels) that are cached when the list is in view. By default, QML deletes any delegtes from memory when they are not in view. Setting a cache buffer of 1000 pixels in this case means that 10 items will be cached, so these items (including thumbnails downloaded over the network) do not need to be reloaded when scrolling. A higher cache buffer will actually be much better for slower internet connections.

The 15 list items you refer to is the number of items (count - 15) before grabbing further results (the API returns a maximum of 50 at any one time). In other words, when scrolling, results are grabbed once you get to 15 items before the end of the list.