The SlippyMap extension requires a small change to the original OpenLayers code. At the very end of the file, it sets a variable to "true", so that the JavaScript can be loaded asynchronously. The part of the JavaScript that creates the map polls for these variables. If they are not yet set, the function calls itself using alarm() and a ~0.5s delay. Without this hack, the loading of the page would stall at the map in many browsers when using slow links or slow CPUs