Honestly I've never had that issue, or never noticed it I guess. Shadow Boost seems to help but my framerate is always staying above 40 without any dips. Doesn't matter where I am either. I also haven't noticed any objects disappearing, I just make sure there's no obvious conflicts in the mods I install and install any compatibility patches they might come with, and then there's no issue. I didn't add a 'delete everything from the world' partly for that reason. I usually have a lot of issues on other games, for example Fallout 3 is just full of huge red triangles and un-UV mapped textures over every object, but Fallout 4 seems pretty stable.