It seems the original save is now bloated and causing the lag. I have been searching for any sort of fix for this. I found one source that said by interacting with the environment you are causing those bits to change. While constantly doing this across the whole map it is trying to remember everything you have affected. Another said to try waiting for 30 days in game to clear out caches. No luck here.
I would prefer to go through everything completely with the single character. If I really have to I can load the save just before Dead Money each time I want to do one of the DLCs. Though it seems completely reasonable to be able to go through a game and do every available quest in a single playthrough without breaking the game.
Any help with this would be wonderfully appreciated.
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System specs:
Windows 7 Professional 64-bit
BIOS Date: 02/05/10 19:13:52 Ver: 08.00.10
Intel® Core i7-2600K CPU @ 3.40GHz (8 CPUs), ~3.4GHz
16384MB RAM
NVIDIA GeForce GTX 570 - 295.51 drivers
2560 x 1600 (32 bit) (60Hz)