I remember when i played FO3 on PS3 years ago, the constant freezing near the end of the campaign was brutal. i almost gave up, but managed to complete it..
NV was also a freeze fest, but maybe not quite as bad. But i think the freezing and how quickly it begins all depends on how much you interact with the world in these games. It saves everything you do, every door you open, item you drop, every tin can on the ground you bump into and move, etc. Save file gets really big and the freezing starts, its inevitable from my experience with these games on PS3.