Possible related things
1) set Shadows, Effects, Foliage, to low (0) - if you can't keep the game up long enough to do that, there's another post on here that I made with instructions on how to work around that. View distance may, as well as the others, help too, setting them lower.
2) have you cut down a fair number of trees, or something? it could be related, there's a method to nerf the duplicated trees, but its a little more complex
3) resolution/settings of computer might be one thing, but more game-related, there's also the blueprint near the greatsword display - it can duplicate and stack itself, they may cause lag for you, maybe. - You can take them to the incinerator and burn them if you think they're related