SYMPTOMS. In my case, the symptomatic behavior I see is a cumulative slowdown whenever transitioning into a busy area or interacting with merchants. That is, with each transition the game gets slower and in fact I can intentionally crash by transitioning between the strip and a casino several times in a row. If I leave my character standing in the strip for a long time (overnight real time), it will crash, I believe because of the dancers and other NPCs that cycle in and out of the area. Once it starts to noticeably slow, you might get only 2 or 3 more transitions before it will become unresponsive. The disk thrashes when it slows. Similar to Fallout 3, it seems to have gotten much worse with the DLCs.
POSSIBLE CAUSE. The degrading performance behavior of the game can be fitted to a memory leak model, one of the most common software bugs. When you leave an area or as NPCs may cycle in and out of range around you, the game does not release memory properly. The slowing down and disk thrashing may be one in the same and can be explained by the fact that many systems and applications, and surely New Vegas and PS3 OS do, make use of memory swap files. Swap files are just portions of disk that appear as extra RAM to the game, managed by the OS or low level software layer, so the game thinks it has a huge amount of RAM available. As the memory leak(s) gradually rob the system of physical RAM, all that is left is the swap file, so the game/app/OS will begin to thrash the disk as it frantically moves memory data between the swap file and the tiny amount of RAM left. Hard disks are much slower than RAM so this would in effect slow the game to "hard disk speed" from RAM speed. Of course, once the RAM is depleted it freezes. This would also explain why people who put the graphics on 480p and other low res options report better mean time between slow-ups, but ultimately still had them. The lower res saved more memory but the leak is still there, it just took longer to deplete the bigger pool of the RAM. Just a theory.
WHY NOT FIXED? Bethesda never fixed it in Fallout 3 so I don't see this one getting fixed. While memory leaks can be tricky to find, they can be found 100% of the time and corrected. Since Bethesda is able to and does make software changes and patches, I would guess this issue is in code they do not have rights or ability to modify (like in a purchased physics or graphics engine.... ). Or perhaps it is a lower level code they wrote that they feel is too risky or expensive to modify.
WHY HAPPENING TO SOME BUT NOT OTHERS? That is the big question. I have a PS3 160Gb model (maybe 3 yrs old), playing on 1080p, using optic audio out. All DLCs, 1.7 Patch, latest PS3 update, 6Mb file. My playing styles that could be considered suspect for causing glitching would be: I hoard, especially with weapons, ammo and upgrades and I do the casino chip caps glitch at some point (no time to grind). So one theory might be that having lots of inventory, several million caps, or just having bought a lot of stuff causes it. I stopped hoarding and put most of what I had into storage, but it did not reverse the FPS slowdowns. I have another play through on the system currently at level 10, no glitching, small inventory and few saves. So far not much problem, although that one just made the Strip. We'll see what happens. That is when the trouble started for my other characters. You can do a simple test, just repeatedly transition from a casino to strip and back again and see if it slows down. Mine will go maybe 10 transitions before unresponsive. If some people can do that without adverse affects, that would be interesting to know.
WORKAROUND: When the FPS is less than 1 every second or so, save, quit game, restart, and continue. It doesn't take nearly as much time as a hard freeze where you have to reset or power cycle the PS3.
It will be disappointing if I can't finish the DLCs but I have had a lot of fun so far. With Fallout 3, I had to give up on a couple of the DLCs. The one thing I am concerned about is Skyrim. I hope whatever disease is in this game, it did not go into Skyrim.
Disclosure: I code for a living (not games).