This is a static, non-interactive, version of prettymaps.
That means that all the different image layers (OSM, Flickr, Natural Earth) have been combined in to a single layer and none of the data layers, containing places names and outlines, are being sent to the browser.
It's basically a simple slippy map
to demonstrate the pretty in prettymaps.
Big chunks of prettymaps have been left un-optimized by design because we wanted to leave all the plumbing exposed so that people could look at it and learn from it and, hopefully, build something new. This version was added because the default version of prettymaps can be difficult to view for people with older computers or slow network connections and can, hopefully, act as a kind of middle ground between the fast and the possible.
The full, interactive, version of prettymaps is over here.