At the lowest graphics setting, this game is lagging. Now if this was Infinity Engine, I would just choose a lower resolution, but here it starts at 1280x720. Any idea?

Notebook Intelcore Duo CPU 8700 @ 2,53 GHz
8GB RAM

Of course that's no State-of-the-art gaming machine, but lowering the resolution did it for me in most of the cases when there was lagging. E.g. I would not run Shadowrun Dragonfall on it's highest possible resolution, or NWN2, but then it all works fine.

Now these days even my cell phone has a higher resolution than older PCs from the 90s had. That's a pity, because it seems that I have been taken away the possibility of choosing the lower resolution.

hmm yeah sounds like your notebook is too weak for the game, my guess is the graphics card.
Maybe there will be a patch or mod for lower resolution.
I don't blame the developers though, the game looks very good and is a true pierce of Art, i hope you get to play it soon.