In the in-game config, have you set the FPS to 30, turned off shadows, and turned off "Extended drawing"? Until recently I had to do this in specific areas that were incredibly laggy if they had a lot of trees. It's really the only thing to do and if you do have these settings as low as possible your PC unfortunately just probably can't run the current game. Keep an eye out for 2.0 which is going to have a much more efficient/low-end-friendly engine.