sebi,
I've been looking at your sample, and I get an average of 10-20 fps on my system.
The problem I can see though appears to be not to do with the drawing, but of the calculations for your isometric map viewing.
I'm going to experiment with this a bit because I think that the co-ordinate calculations can be optimized by using a lookup table instead of calculating every frame. This will certainly improve the speed, but there may be other areas of the main routine that I've not yet determined where the bottle necks are.
Interesting concept though, I can see what you're trying to do.
Paul.