COLGeek: Well, I don´t have chance to do that, my old laptop is broken. I have the latest version of Optifine, and when I use the multicore-chunkloading, it runs better, but in singleplayer, world flickers, and I haven´t disable "Threaded Optimization" / "OpenGL Threading" in the graphics card control panel, because I haven´t have time for that. That could help, I guess.
JeremyRamsey: Like I said before this, I use the latest Optifine, and I have increased my RAM in Minecraft. Minecraft uses about half of my RAM now, and it has done it slightly better and smoother, but I still can´t use good settings in that world.
Thank you both anyway!