FNV: 60fps in old saves but 30 in newer saves in same area

[robgrab]

I'm having a problem in FNV where I was playing through the Old World Blues DLC using the Enhanced Shaders ENB (ultra high quality preset) getting a rock sold 60fps both indoors and outdoors. As soon as I teleported back to the main Vegas wasteland map my frame rate started to drop, which I expected since the area is much larger. But the frame rate slowly continued to decline over time to where it barely runs at 30fps now. I just figured it's because the area was so much larger so I transported back to the Big MT map in Old World Blues and now the frame rate was slow there too! I went inside The Sink, which is a very small living area, and the frame rate was barely hitting 30fps. So I loaded a previous save game in the exact same room and it was back to 60fps! I even walked back outside and it was still 60fps. What happened?! I know that Bethesda save games tend to get bloated, especially since this was save number 1246. But the previous game I loaded was only a few hours earlier and I've been playing the game over 100 hours. Is there some sort of memory leak going on that can be fixed or purged? Any suggestions?

NOTE: It doesn't matter if the ENB is on or off. It's 30fps regardless.

UPDATE: Running an Archive Invalidation fixed everything.