On a UA 747 from LHR to SFO. Somewhere over Greenland I could feel us descending and the moving map stopped working. A good twenty minutes later the Captain announced a pressurization issue limiting us to IIRC 14000 feet. He said we had to fly low and slow. We couldn't make it all the way to SFO so we were diverting to ORD - which took as long as flying all the way to SFO.