I think you've nailed it there. I'm running with 32 GB of RAM, and when LR is chugging away I've seen the resource monitor go up as high as 16 GB usage. So when your system starts to top out at 8 GB, I'm guessing it begins to use the hard drive instead. Or maybe it just throttles back the processing to keep within the RAM limit. If your system can handle 16 GB RAM, it would probably be your most cost-effective upgrade.