An Alaska Airlines Boeing 737-900, registration N305AS performing flight AS-930 from Seattle,WA to Las Vegas,NV (USA), was climbing out of Seattle when the crew stopped the climb at FL250 due to problems with the cabin pressure but continued the flight into the direction of Las Vegas. About 17 minutes later the crew initiated an emergency descent to 10,000 feet due to the loss of cabin pressure and diverted to Portland,OR (USA) for a safe landing about one hour after leaving FL250. A passenger reported the aircraft performed an emergency descent due to the "loss of compression" and diverted to Portland.