Cached layers page may take minutes to show up (when backing stores are not responsive)

The cached layer page (and to some extent, the preview page) can get stuck generating the page for a long time, while computing the type of resource backing it (e.g., point, line ,polygon) if the backing store is slow to respond.
It would be best to return the page immediately, and then compute the type resource by resource, delaying at most the appearance of one of the icons in the page, rather than the page itself.