Maybe your xbmc installation somehow got corrupt as its supposed to perform quite smoothly on windows and your hardware is more than sufficient (Try clean uninstall & reinstall?). As for buffering, you using Ethernet or WIFI? Also depending on the url you are streaming from, you might want to try streaming just 720P and below, cuz some streaming urls like "beststreams", "Gorrillavids", "nowvideo" etc, they serve out media much better and faster.