@beaguest
There was indeed something wrong with the Windows installation, I reinstalled it and some things changed,
but the average FPS are still the same on default graphic settings. I ordered the CPU now to give it a try.
@Haunty
I tried it. With the new installation, I get now about 70-80 FPS with deactivated shadows, but that looks creepy.
I guess, I have to live with it.